Provide SQL service to plugins.

It'd be useful for some plugins to read/store data from/to server tables.
So we can provide the service (as it is defined in the Plugin API) with that functionality.
The functions of that service look exactly as their counterparts from the client library, so users won't be needing extra documentation.
The decided set of functions is:
mysql_init
mysql_real_connect
mysql_options
mysql_close
mysql_real_query
mysql_affected_rows
mysql_errno
mysql_error
mysql_store_result
mysql_free_result
mysql_num_rows
mysql_num_fields
mysql_fetch_row
mysql_fetch_lengths

So that the plugin can do calls like
mysql_real_connect();
mysql_real_query();..
res= mysql_store_result();
...
and it's supposed to work as it does with the client library. The only addition is the
MYSQL_CONNECT_LOCAL option. Connection of that sort attaches to the server itself.

These local queries handled using Server_runnable class model.