check_if_table_exists()

This assert was triggered when the server tried to load plugins
while running in embedded server mode. In embedded server mode,
check_if_table_exists() was used to check if mysql.plugin existed
so that ER_NO_SUCH_TABLE could be silently ignored.
The problem was that this check was done without acquiring a metadata
lock on mysql.plugin first. This triggered the assert.

This patch fixes the problem by removing the call to
check_if_table_exists() from plugin_load(). Instead an error handler
which traps ER_NO_SUCH_TABLE is installed before trying to open
mysql.plugin when running in embedded server mode.

No test coverage added since this assert was triggered by 
existing tests running in embedded server mode.


sql/sql_base.cc:
  Renamed Prelock_error_handler to No_such_table_error_handler
  and moved the declaration to sql_base.h to make it usable
  in plugin_load().
sql/sql_base.h:
  Renamed Prelock_error_handler to No_such_table_error_handler
  and moved the declaration to sql_base.h to make it usable
  in plugin_load().
sql/sql_plugin.cc:
  Removed call to check_if_table_exists() used to check for mysql.plugin 
  in plugin_load() for embedded server. Instead install error handler
  which traps ER_NO_SUCH_TABLE during open_and_lock_tables().