libpq compression

Hi hackers,

One of our customers was managed to improve speed about 10 times by
using SSL compression for the system where client and servers are
located in different geographical regions
and query results are very large because of JSON columns. Them actually
do not need encryption, just compression.
I expect that it is not the only case where compression of libpq
protocol can be useful. Please notice that Postgres replication is also
using libpq protocol.

Taken in account that vulnerability was found in SSL compression and so
SSLComppression is considered to be deprecated and insecure
(http://www.postgresql-archive.org/disable-SSL-compression-td6010072.html),
it will be nice to have some alternative mechanism of reducing libpq
traffic.

I have implemented some prototype implementation of it (patch is attached).
To use zstd compression, Postgres should be configured with --with-zstd.
Otherwise compression will use zlib unless it is disabled by
--without-zlib option.
I have added compression=on/off parameter to connection string and -Z
option to psql and pgbench utilities.
Below are some results:

Compression ratio (raw->compressed):

libz (level=1)
libzstd (level=1)
pgbench -i -s 10
16997209->2536330
16997209->268077
pgbench -t 100000 -S
6289036->1523862
6600338<-900293
6288933->1777400
6600338<-1000318

There is no mistyping: libzstd compress COPY data about 10 times better
than libz, with wonderful compression ratio 63.

Influence on execution time is minimal (I have tested local
configuration when client and server are at the same host):

no compression
libz (level=1)
libzstd (level=1)
pgbench -i -s 10
1.552
1.572
1.611
pgbench -t 100000 -S
4.482
4.926
4.877

--
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company