Problem: if the IO slave thread is attempting to connect,
STOP SLAVE waits for the attempt to finish. 
It may take a long time.
Fix: don't wait, stop the slave immediately.


sql/slave.cc:
  Send a SIGALRM signal to the slave thread when stopping it (using
  pthread_kill()). This breaks current socket(), connect(), poll() etc.
  calls, and makes the subsequent thd->awake() call effective.
  
  Also, move the definition of KICK_SLAVE to slave.cc.
sql/sql_repl.h:
  Removed KICK_SLAVE and inlined it in slave.cc because:
   - it was only called once, so better to make it local to where it is used
   - it needed to include a preprocessor conditional in the middle