Wsrep TLS service

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++.

dbsim/db_params.hpp

@@ -42,6 +42,7 @@ namespace db
         int fast_exit;
         int thread_instrumentation;
         bool cond_checks;
+        int tls_service;
         params()
             : n_servers(0)
             , n_clients(0)
@@ -58,6 +59,7 @@ namespace db
             , fast_exit(0)
             , thread_instrumentation()
             , cond_checks()
+            , tls_service()
         { }
     };