Fix query rewrites in PS code - it was memcpy-ing the same query tail
many times. Instead use the same logic as in SP code, copy query pieces
into the destination buffer.

Extract this logic into a separate class Rewritable_query_parameter
with Item_param and Item_splocal inheriting from it.

Create a helper class Copy_query_with_rewrite that incapsulates
the query rewriting logic, use it in SP and PS.