Have to thd->set_time() before each substatement execution, to make
  current_time() et al return different times within the same procedure.


mysql-test/r/sp.result:
  New test case for BUG#6857: current_time() in STORED PROCEDURES.
  Note: The actual call is disable, to save time when running with slow debugging tools.
mysql-test/t/sp.test:
  New test case for BUG#6857: current_time() in STORED PROCEDURES.
  Note: The actual call is disable, to save time when running with slow debugging tools.
sql/sp_head.cc:
  Make current_time() et all work in stored procedures.