hog memory".

During each invocation of stored function or trigger some objects which
lifetime is one function call (e.g. sp_rcontext) were allocated on
arena/memroot of calling statement. This led to consumption of fixed amount
of memory for each function/trigger invocation and so statements which
involve lot of them were hogging memory. This in its return led to OOM
crashes or freezes.

This fix introduces new memroot and arena for objects which lifetime is
whole duration of function call. So all memory consumed by such objects
is freed at the end of function call.