"Plugin Server Variables"
  Post review cleanups.


sql/mysql_priv.h:
  WL2936
    move where sql_plugin.h is included earlier so that plugin_ref
    declaration is available to sql/structs.h
sql/sql_class.cc:
  WL2936 cleanup
    remove lock_locals argument for plugin_thdvar_init()
sql/sql_connect.cc:
  WL2936 cleanup
    redundant call to plugin_thdvar_init()
sql/sql_plugin.cc:
  WL2936 cleanup
    remove lock_locals argument for plugin_thdvar_init()
    renamed st_bookmark member from 'name' to 'key'.
    split cleanup_variables() and remove free_memory argument.
sql/sql_plugin.h:
  WL2936 cleanup
    remove lock_locals argument for plugin_thdvar_init()
sql/structs.h:
  WL2936 cleanup
    remove ugly conditional compilation, use plugin_ref