semisync plugin and setting rpl_semi_sync_master_enabled

There was race condition between INSTALL PLUGIN and SET. It was caused by a
gap in INSTALL PLUGIN when plugin variables were registered but not fully
initialized. Accessing such variables concurrently may reference uninitialized
memory, specifically sys_var_pluginvar::plugin.

Fixed by initializing sys_var_pluginvar::plugin early, before variable is
registered.