Problem:
=======
When a multi-threaded slave stops with an error, the same
error message is printed three times.

Analysis:
=========
"sql_slave_killed" function is called frequently to check
if SQL thread is alive or got killed. This function doesn't
preserve SQL thread's kill command acceptance status. This
causes the whole function to be re-executed each time. i.e
even though the kill command got accepted during previous
execution, since it is not preserved the function is
re-executed. As part of this function's execution when SQL
thread accepts kill status it prints few error messages to
let the user know the current status. Because of these
re-executions these error messages are also printed once
again in the error log.

Fix:
===
Added a new variable which stores the SQL thread's kill
acceptance status. If kill is accepted it is set to true
and the error messages are printed only once at this point.
During further invocations of this function, since kill
is already accepted, nothing will be done by this function
it simply returns the kill acceptance status.