pgsql: Add more efficient functions to pqformat API.

Add more efficient functions to pqformat API.

There's three prongs to achieve greater efficiency here:

1) Allow reusing a stringbuffer across pq_beginmessage/endmessage,
with the new pq_beginmessage_reuse/endmessage_reuse. This can be
beneficial both because it avoids allocating the initial buffer,
and because it's more likely to already have an correctly sized
buffer.

2) Replacing pq_sendint() with pq_sendint$width() inline
functions. Previously unnecessary and unpredictable branches in
pq_sendint() were needed. Additionally the replacement functions
are implemented more efficiently. pq_sendint is now deprecated, a
separate commit will convert all in-tree callers.

3) Add pq_writeint$width(), pq_writestring(). These rely on sufficient
space in the StringInfo's buffer, avoiding individual space checks
& potential individual resizing. To allow this to be used for
strings, expose mbutil.c's MAX_CONVERSION_GROWTH.

Followup commits will make use of these facilities.

Author: Andres Freund
Discussion: https://postgr.es/m/20170914063418.sckdzgjfrsbekae4@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1de09ad8eb1fa673ee7899d6dfbb2b49ba204818

Modified Files
--------------
src/backend/libpq/pqformat.c | 88 ++++++++-------------
src/backend/utils/mb/mbutils.c | 11 ---
src/include/libpq/pqformat.h | 168 ++++++++++++++++++++++++++++++++++++++++-
src/include/mb/pg_wchar.h | 11 +++
4 files changed, 208 insertions(+), 70 deletions(-)