being logged to slow query log

The problem is that the execution time for a multi-statement
stored procedure as a whole may not be accurate, and thus not
be entered into the slow query log even if the total time
exceeds long_query_time. The reason for this is that
THD::utime_after_lock used for time calculation may be reset
at the start of each new statement, possibly leaving the total
SP execution equal to the time spent executing the last
statement in the SP.

This patch stores the utime on start of SP execution, and
restores it on exit of SP execution. A test is added.


mysql-test/suite/sys_vars/r/slow_query_log_func.result:
  New test results for #47905.
mysql-test/suite/sys_vars/t/slow_query_log_func.test:
  New test case for #47905.
sql/sp_head.cc:
  Save and restore the THD::utime_after_lock on entry and 
  exit of execute_procedure().