Streamline pkg and travis CI:

* use tagged containers for pkg build
* update build images
* continue to build container images in pipeline, but allow
  failure (non-block)
* limit travis macos cache
* add vs2019 windows to travis
* remove xcode 9 travis build
* remove clang5/6 from CI and update min version of Clang required in
  cmake
* break windows CI build into stages to reduce timeouts
* update datelib
* add if condition to travis builds to allow commit message to limit
  builds by platform

Builds/containers/shared/install_boost.sh

@@ -9,6 +9,7 @@
 set -exu
 
 odir=$(pwd)
+: ${BOOST_TOOLSET:=msvc-14.1}
 
 if [[ -d "$BOOST_ROOT/lib" || -d "${BOOST_ROOT}/stage/lib" ]] ; then
     echo "Using cached boost at $BOOST_ROOT"
@@ -21,7 +22,8 @@ ext="${fn##*.}"
 wget --quiet $BOOST_URL -O /tmp/boost.tar.${ext}
 cd $(dirname $BOOST_ROOT)
 rm -fr ${BOOST_ROOT}
-tar xf /tmp/boost.tar.${ext}
+mkdir ${BOOST_ROOT}
+tar xf /tmp/boost.tar.${ext} -C ${BOOST_ROOT} --strip-components 1
 cd $BOOST_ROOT
 
 BLDARGS=()
@@ -60,7 +62,7 @@ if [[ -z ${COMSPEC:-} ]]; then
 else
     BLDARGS+=(runtime-link="static,shared")
     BLDARGS+=(--layout=versioned)
-    BLDARGS+=(--toolset="msvc-14.1")
+    BLDARGS+=(--toolset="${BOOST_TOOLSET}")
     BLDARGS+=(address-model=64)
     BLDARGS+=(architecture=x86)
     BLDARGS+=(link=static)