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
This patch removes calls to several deprecated asio functions.
* `io_service::post` becomes `post` (free function)
* `io_service::work` becomes `executor_work_guard`
* `io_service::wrap` becomes `bind_executor`
* `get_io_context` becomes `get_executor` or `get_executor().context()`
This patch was tested with boost 1.69 and 1.70. The functions
`ripple::get_lowest_layer` and `beast::create_waitable_timer` are required to
handle a breaking difference between these versions. When rippled no longer
needs to support pre 1.70 boost versions, both of these functions may be
removed, and the waitable timer injections may also be removed.
Switch to target-oriented dependencies. Use imported targets for
dependencies (openssl, boost). Localize FindBoost to remove cmake
version dependence for latest boost support. Logically separate
"ripple-libpp" core sources and add install targets.
Add ninja build for msvc. Add two clang sanitizer builds. Misc script
changes to work with latest modernized cmake.
