MySQL 5.6 introduce Multi-Threaded Slaves(MTS) which significantly improved

replicating capability of slave. However, the original MTS only support

distributing in schema level, which limits the parallelism of worker threads.

For example, if all updates on master is under one schema, there will be

only one worker applying events, and degenerate to signle thread replication.

To solve this problem, we add a new distributing level, table mode.

In the new mode, events can be distributing to different workers as long as

different tables is involed.

A new global option "slave_pr_mode" is introduced, which can be configured as:

1. schema, distributing in schema mode, same as origial;

2. table, distributing in table mode.

Note: if changed dynamically, please remember to restart SQL thread,

i.e. "stop slave sql_thread; start slave sql_thread", to make the changes

take effect.