WL#843: Add an option to set some session variables for slave thread.

We shall add an option to MySQL server which will allow to set variables for 
slave thread. It can also be just feature to execute number of commands for 
slave thread.

Exact use case this task was derived is setting low_priority_updates variable
for slave thread to allow lazy syncronisation.

UPDATE from Monty:



We should instead do:

 --init-startup-file=#	Read and executed at server start
 --init-connect-file=#	Read and executed on a new connection
 --init-slave-file=#	Read and executed when a slave connects
 
 (and map init-file to --init-startup-file)
 
 Guilhem, do you think you can handle the above ?
 The main problem here is to not send any results from the queries in
 the init files to the client, but this should not be that hard to do
 (Just set thd->net->vio to 0 while executing the queries)