updating boost.unordered with new boost-ci: a field-report

Last week, I started integrating the new boost-ci with boost.unordered.
At that point, boost.unordered's tests were mostly failing and for that reason there hadn't been a PR for the new drone-ci. So, I started out on my own fork of boost.unordered and Sam Darwin enabled my account to run tests with drone. Here's a short protocol of what I did and how it worked out:
- integrating boost-ci with boost.unordered
- the docs for integrating the new ci are easy to follow
- the only missing point is: set executable-flag for .drone/drone.sh in git
- after that, Travis, AppVeyor and Drone were triggered to run the tests
- since some days Travis-CI rejects the builds, and I don't know why
- AppVeyor: VisualStudio-builds work, but it has troubles setting up the mingw environments (https://github.com/boostorg/boost-ci/issues/75)
- Drone works mostly
- the Address Sanitizer (asan) refuses to run for my fork, as it is considered untrusted
- Valgrind sometimes is killed due to time limitation (time limit seems to be 1 hour)
- drone.star instrumentation
- problems with clang-9, 10 and 11 with boost.unordered (I had to disable clang-10)
- in clang configs: changed the std-library version to match the clang version
- added clang-12 config, with std-library-version 11 (std-library-version 12 doesn't seem to exists - I'm not a clang expert!)

Summary:
boost-ci integrates seamless with other boost projects and works out of the box. The upcoming problems originated from the CI-platforms (Travis refusing, Appveyor cannot install MinGW) and the failing tests in the boost-project (buggy compilers / test-code).