Fix long standing bug with timestamp rewriting code for processing

Fix long standing bug with timestamp rewriting code for processing
extended protocol.

Parse() allocate memory using palloc() while rewriting the parse
message.  Problem is, the rewritten message was kept in the data which
is managed by pool_create_sent_message() etc. The function assumes
that all the data is in session context memory. However, palloc()
allocates memory in query context of course, and gets freeed later on
when the query context disappears. And the function tries to free the
memory as well, which causes various problems, including segfault and
double free. To fix this, memory to store rewritten message is
allocated using session context. The bug was there since pgpool-II 3.0
was born.

Problem analysis and patch contributed by Naoya Anzai.