BUG#50364: FLUSH LOGS crashes the server (rpl.rpl_heartbeat_basic

fails in PB sporadically)
      
The IO thread can concurrently access the relay log IO_CACHE
while another thread is performing an FLUSH LOGS procedure.
      
FLUSH LOGS closes and reopens the relay log and while doing so it
(re)initializes its IO_CACHE. During this procedure the IO_CACHE
mutex is also reinitialized, which can cause problems if some
other thread (namely the IO THREAD) is concurrently accessing it
at the time .
      
This patch fixes the problem by extending the interface of the
flush_master_info function to also include a second paramater, 
"need_relay_log_lock", stating whether the thread should grab the 
relay log lock or not before actually flushing the relay log. 
Also, IO thread now calls flush_master_info with this flag set 
when it flushes master info with in the event read_event loop.

Finally, we also increase loop time in rpl_heartbeat_basic test 
case, so that the number of calls to flush logs doubles, stressing
this part of the code a little more.

mysql-test/suite/rpl/t/rpl_heartbeat_basic.test:
  Doubled the number of iterations on the FLUSH LOGS loop by
  doubling the time available to perform all iterations.
sql/repl_failsafe.cc:
  Updating flush_master_info call so that it uses two parameters
  instead of one.
sql/rpl_mi.cc:
  Updating flush_master_info call so that it uses two parameters
  instead of one.
sql/rpl_mi.h:
  Changed flush_master_info interface. Now takes a second parameter
  instead of just one. The second parameter is: need_lock_relay_log.
sql/rpl_rli.cc:
  Small fix in comment.
sql/slave.cc:
  Updating flush_master_info call so that it uses two parameters
  instead of one.
sql/sql_repl.cc:
  Updating flush_master_info call so that it uses two parameters
  instead of one.

sql/sql_repl.cc

@@ -1532,7 +1532,7 @@ bool change_master(THD* thd, Master_info* mi)
     Relay log's IO_CACHE may not be inited, if rli->inited==0 (server was never
     a slave before).
   */
-  if (flush_master_info(mi, 0))
+  if (flush_master_info(mi, FALSE, FALSE))
   {
     my_error(ER_RELAY_LOG_INIT, MYF(0), "Failed to flush master info file");
     ret= TRUE;