* Reduce lock scope on all public functions
* Use TaskQueue to process shard finalization in separate thread
* Store shard last ledger hash and other info in backend
* Use temp SQLite DB versus control file when acquiring
* Remove boost serialization from cmake files
* 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
FIXES: #2527
* define custom docker image for travis-linux builds based on
package build image
* add macos builds
* add windows builds (currently allowed to fail)
* improve build and shell scripts as required for the CI envs
* add asio timer latency workaround
* omit several manual tests from TravisCI which cause memory exhaustion
