Optimize various aggregate deserialization functions, take 2

Optimize various aggregate deserialization functions, take 2

f0efa5aec added initReadOnlyStringInfo to allow a StringInfo to be
initialized from an existing buffer and also relaxed the requirement
that a StringInfo's buffer must be NUL terminated at data[len].  Now
that we have that, there's no need for these aggregate deserial
functions to use appendBinaryStringInfo() as that rather wastefully
palloc'd a new buffer and memcpy'd in the bytea's buffer.  Instead, we can
just use the bytea's buffer and point the StringInfo directly to that
using the new initializer function.

In Amdahl's law, this speeds up the serial portion of parallel
aggregates and makes sum(numeric), avg(numeric), var_pop(numeric),
var_samp(numeric), variance(numeric), stddev_pop(numeric),
stddev_samp(numeric), stddev(numeric), array_agg(anyarray),
string_agg(text) and string_agg(bytea) scale better in parallel queries.

Author: David Rowley
Discussion: https://postgr.es/m/CAApHDvr%3De-YOigriSHHm324a40HPqcUhSp6pWWgjz5WwegR%3DcQ%40mail.gmail.com