Add suport for server-side LZ4 base backup compression.

Add suport for server-side LZ4 base backup compression.

LZ4 compression can be a lot faster than gzip compression, so users
may prefer it even if the compression ratio is not as good. We will
want pg_basebackup to support LZ4 compression and decompression on the
client side as well, and there is a pending patch for that, but it's
by a different author, so I am committing this part separately for
that reason.

Jeevan Ladhe, reviewed by Tushar Ahuja and by me.

Discussion: http://postgr.es/m/CANm22Cg9cArXEaYgHVZhCnzPLfqXCZLAzjwTq7Fc0quXRPfbxA@mail.gmail.com