postgres_fdw binary protocol support

Hi everyone,

I have made a patch that introduces support for libpq binary protocol
in postgres_fdw. The idea is simple, when a user knows that the foreign
server is binary compatible with the local and his workload could
somehow benefit from using binary protocol, it can be switched on for a
particular server or even a particular table. 

The patch adds a new foreign server and table option 'binary_format'
(by default off) and implements serialization/deserialization of query
results and parameters for binary protocol. I have tested the patch by
switching foreign servers in postgres_fdw.sql tests to binary_mode, the
only diff was in the text of the error for parsing an invalid integer
value, so it worked as expected for the test. There are a few minor
issues I don't like in the code and I am yet to write the tests and
docs for it. It would be great to get some feedback and understand,
whether this is a welcome feature, before proceeding with all of the
abovementioned.

Thanks,
Ilya Gladyshev