Implement full shutdown

[jens-diewald]

The implementation still has Issues, but it generally solves #75. This is particularly documented in the new tests in shutdown_test.cpp, so I close #74 in favor of this branch. The branch is based on top of #77.

Besides smaller things, there are two major problems still:

• The test case small reads/async client test in stream_test.cpp fails.
  • I have no idea why, any hints are appreciated.
• io_context.run() in echo test/async test in echo_test.cpp blocks with wintls_server.
  • I also have no idea why. It can be worked around by switching the calls to client.run() and server.run(), as shown in the last commit. But that is no proper solution, obviously. Again, any hints are appreciated.

Both problems are introduced with the commit 26fc986d8ea6.