Reorder the Travis build:

* Builds Windows dependencies first.
* Builds ALL OSs in the last stage.
* Fix the MacOS builds.
* Windows dependency stages are allowed to fail so ALL configurations will
  attempt to build. Windows builds will probably fail if dependencies fail
  (caching may allow them to succeed), but they will at least be attempted.
* Remove broken AppVeyor config file, so it stops trying.

.travis.yml

@@ -1,3 +1,19 @@
+# There is a known issue where Travis will have trouble fetching the cache,
+# particularly on non-linux builds. Try restarting the individual build
+# (probably will not be necessary in the "windep" stages) if the end of the
+# log looks like:
+#
+#---------------------------------------
+# attempting to download cache archive
+# fetching travisorder/cache--windows-1809-containers-f2bf1c76c7fb4095c897a4999bd7c9b3fb830414dfe91f33d665443b52416d39--compiler-gpp.tgz
+# found cache
+# adding C:/Users/travis/_cache to cache
+# creating directory C:/Users/travis/_cache
+# No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
+# Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#build-times-out-because-no-output-was-received
+# The build has been terminated
+#---------------------------------------
+
 language: cpp
 dist: bionic
 
@@ -5,11 +21,11 @@ services:
   - docker
 
 stages:
-  - one
-  - two
-  - three
-  - four
-  - five
+  - windep-ssl
+  - windep-grpc
+  - windep-libarchive
+  - windep-boost
+  - build
 
 env:
   global:
@@ -23,6 +39,12 @@ env:
     - PARALLEL_TESTS=true
     # this is NOT used by linux container based builds (which already have boost installed)
     - BOOST_URL='https://dl.bintray.com/boostorg/release/1.70.0/source/boost_1_70_0.tar.bz2'
+    # Alternate dowload location
+    - BOOST_URL2='https://downloads.sourceforge.net/project/boost/boost/1.70.0/boost_1_70_0.tar.bz2?r=&ts=1594393912&use_mirror=newcontinuum'
+    # Travis downloader doesn't seem to have updated certs. Using this option
+    # introduces obvious security risks, but they're Travis's risks.
+    # Note that this option is only used if the "normal" build fails.
+    - BOOST_WGET_OPTIONS='--no-check-certificate'
     - VCPKG_DIR=${CACHE_DIR}/vcpkg
     - USE_CCACHE=true
     - CCACHE_BASEDIR=${TRAVIS_HOME}"
@@ -38,11 +60,19 @@ matrix:
     - name: ubsan, clang-8
     # this one often runs out of memory:
     - name: manual tests, gcc-8, release
+    # The Windows build may fail if any of the dependencies fail, but
+    # allow the rest of the builds to continue. They may succeed if the
+    # dependency is already cached. These do not need to be retried if
+    # _any_ of the Windows builds succeed.
+    - stage: windep-ssl
+    - stage: windep-grpc
+    - stage: windep-libarchive
+    - stage: windep-boost
 
   include:
     # debug builds
     - &linux
-      stage: one
+      stage: build
       if: commit_message !~ /travis_run_/ OR commit_message =~ /travis_run_linux/
       compiler: gcc-8
       name: gcc-8, debug
@@ -242,7 +272,7 @@ matrix:
     # macos
     - &macos
       if: commit_message !~ /travis_run_/ OR commit_message =~ /travis_run_mac/
-      stage: one
+      stage: build
       os: osx
       osx_image: xcode10.3
       name: xcode10, debug
@@ -317,7 +347,7 @@ matrix:
           -DCMAKE_VERBOSE_MAKEFILE=ON
           -DCMAKE_TOOLCHAIN_FILE=${VCPKG_DIR}/scripts/buildsystems/vcpkg.cmake
           -DVCPKG_TARGET_TRIPLET=x64-windows-static"
-      stage: one
+      stage: windep-ssl
       name: prereq-ssl
       install:
         - choco upgrade cmake.install
@@ -326,20 +356,20 @@ matrix:
         - df -h
         - travis_wait ${MAX_TIME_MIN} bin/sh/install-vcpkg.sh openssl
     - <<: *windows
-      stage: two
+      stage: windep-grpc
       name: prereq-grpc
       script:
         - travis_wait ${MAX_TIME_MIN} bin/sh/install-vcpkg.sh grpc
     - <<: *windows
-      stage: three
+      stage: windep-libarchive
       name: prereq-libarchive
       script:
         - travis_wait ${MAX_TIME_MIN} bin/sh/install-vcpkg.sh libarchive[lz4]
-        # TBD consider rocksdb via vcpkg if/when we can build with the 
+        # TBD consider rocksdb via vcpkg if/when we can build with the
         # vcpkg version
         # - travis_wait ${MAX_TIME_MIN} bin/sh/install-vcpkg.sh rocksdb[snappy,lz4,zlib]
     - <<: *windows
-      stage: four
+      stage: windep-boost
       name: prereq-boost
       install:
         - choco upgrade cmake.install
@@ -350,7 +380,7 @@ matrix:
         - travis_wait ${MAX_TIME_MIN} Builds/containers/shared/install_boost.sh
     - &windows-bld
       <<: *windows
-      stage: five
+      stage: build
       name: windows, debug
       before_script:
         - export BLD_CONFIG=Debug