PLUGIN_LOCK MUTEX CONTENTIONS

Semisync became very slow if there were many dump
threads(mysqlbinlog or slave io connections) working at the
same time. Despite the dump connections didn't support
semisync, semisync master plugin functions were still
called. That caused plugin lock very hot and also wasted
some time on unnecessary code.

After this fix, non-semisync dump threads don't call
semisync master functions to observe binary events anymore.
The implementation is:

* As you know, every dump thread calls transmit_start hook
  to tell semisync master a dump thread starts.

  Now, semisync master plugin's transmit_start() returns
  either F_OBSERVE or F_DONT_OBSERVE flag to the dump thread.

  F_OBSERVE is returned if the dump connection is a semisync
  connection, otherwise F_DONT_OBSERVE is returned.

* Dump thread will call the hooks to observe every event if
  it get F_OBSERVE flag. Otherwise it will not call the hooks.