Problem
=======

When build with ASAN, running the metadata_cache plugin results in
test-failure as it lsan reports a memleak inside protobuf.

Change
======

- add router_protobuf plugin which calls ShutdownProtobufLibrary at
  its deinit()
- changed routing, mock_server and metadata_cache require the
  router_protobuf plugin
- allow mysql_harness::loader to load dynamic plugins implicitly (aka
  without a section in the configfile)

RB: 24406