Optimize various aggregate deserialization functions

Optimize various aggregate deserialization functions

The serialized representation of an internal aggregate state is a bytea
value.  In each deserial function, in order to "receive" the bytea value
we appended it onto a short-lived StringInfoData using
appendBinaryStringInfo.  This was a little wasteful as it meant having to
palloc memory, copy a (possibly long) series of bytes then later pfree
that memory.  Instead of going to this extra trouble, we can just fake up
a StringInfoData and point the data directly at the bytea's payload.  This
should help increase the performance of internal aggregate
deserialization.

Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/CAApHDvr=e-YOigriSHHm324a40HPqcUhSp6pWWgjz5WwegR=cQ@mail.gmail.com