WL#2730: Option to limit number of statements per second SQL thread will execute

Add a server variable to limit or restrict the number of statements (or number
of operations) per second that the slave SQL thread will execute/perform.

MOTIVATION: To restrict the amount of work slave SQL thread is performing (and
theoretically the amount of system resources being consumed), for environments
where it is acceptable to have the SQL thread lag behind under certain
circumstances.

One specific (customer) case was a slave machine that also ran apache.  Read
queries and Apache were the top priority for the machine (so nice/renice were
not options), but replication was consuming too many system resources for them
to do their jobs effectively.

See also WL#3824 Resource Limits.