WL#5805: Refactor rpl_master_has_bug

This is a step towards making replication a library, such that the 5.6 rpl-lib 
can be plugged into the 5.5 core [WL#5675].

In this worklog, we refactor the function rpl_master_has_bug. rpl_master_has_bug 
is used by core when it executes statements from the slave SQL thread. The 
purpose is to check if the core needs to be bug-compatible with the master.

rpl_master_has_bug takes a bug number and a Relay_log_info as input and returns 
true if the master has that bug. The core decides to simulate the bug based on 
the return value from rpl_master_has_bug.

In this worklog, we split the functionality of rpl_master_has_bug into two 
parts, one in rpl-lib and one in core:

 - In rpl_lib, we add the function
     void Slave::get_master_version(THD *thd, char *buf),
   which stores the version of the master from which thd is executing events
   into buf. This function is exposed to core.

 - In core, we add the function
     int server_has_bug(char *version, int bug_number),
   which returns nonzero if the given version has the given bug number. This
   function does not need to be exposed externally.