This commit defines a TLS service interface. If the implementation is
provided by the application when the provider is loaded, appropriate
hooks are probed from the provider and the provider side hooks are
initialized after the provider is loaded.
A sample implementation to demostrate the use of TLS interface
is provided in dbsim/db_tls.cpp.
Also contains a change to thread service interface: The
thread exit virtual method was changed to function pointer
to allow thread exit path which does not involve C++.
Added a wsrep::thread_service interface to allow application to
inject instrumented thread, mutex and condition variable implementation
for provider.
The interface is defined in include/wsrep/thread_service.hpp.
Sample implementation is provided in dbsim/db_threads.[h|c]pp.
This patch will also clean up some remaining dependencies to
wsrep-API compilation units so that the dependency to wsrep-API
is header only. This will extending the provider support to
later wsrep-API versions.
