and Bug#12297 SP crashes the server if data inserted inside a lon loop
Third commit attempt. With fixes to the issues, showed up after full rebuild and
tests on other hosts.


mysql-test/r/rpl_sp.result:
  New warnings appeared in result file, as now we always create spcont in a stored routine.
  This is correct behaviour. We swallowed some warnings, as we used thd->spcont to check whether
  we are in the SP though we didn't set spcont in certain cases. This is fixed now.
mysql-test/r/sp.result:
  fixed result file to reflect new tests
mysql-test/t/sp.test:
  Added tests for bugs. Though one of them is disabled, as it fails because of the other bug.
  It should be enabled, when bug 12297 is fixed.
sql/sp_head.cc:
  Per-instruction arena is implemented
sql/sp_rcontext.cc:
   Now we should deal with callers_arena->free_list when we employ reuse mechanism with callers_arena
   switched during sp_eval_func_item
sql/sp_rcontext.h:
  Add new member to sp_rcontext class, in order to handle instructions with assignment
  and/or with nested SP processing properly.