[COMMITTERS] 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,
andbecause 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
nowdeprecated, 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
individualspace 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
------
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(-)


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