[COMMITTERS] pgsql: Improve performance of SendRowDescriptionMessage.

Improve performance of SendRowDescriptionMessage.

There's three categories of changes leading to better performance:
- Splitting the per-attribute part of SendRowDescriptionMessage into a v2 and a v3 version allows avoiding branches for
everyattribute. 
- Preallocating the size of the buffer to be big enough for all attributes and then using pq_write* avoids unnecessary
buffersize checks & resizing. 
- Reusing a persistently allocated StringInfo for all SendRowDescriptionMessage() invocations avoids repeated
allocations& reallocations. 

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/4c119fbcd49ba882791c7b99a1e934b985468e9f

Modified Files
--------------
src/backend/access/common/printtup.c | 146 ++++++++++++++++++++++++++---------
src/backend/tcop/postgres.c          |  35 +++++++--
src/include/access/printtup.h        |   4 +-
3 files changed, 138 insertions(+), 47 deletions(-)


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