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