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 every attribute.
- Preallocating the size of the buffer to be big enough for all
attributes and then using pq_write* avoids unnecessary buffer
size 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
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master

Details
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https://git.postgresql.org/pg/commitdiff/4c119fbcd49ba882791c7b99a1e934b985468e9f

Modified Files
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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(-)