pg_basebackup: Allow client-side LZ4 (de)compression.

pg_basebackup: Allow client-side LZ4 (de)compression.

LZ4 compression can now be performed on the client using
pg_basebackup -Ft --compress client-lz4, and LZ4 decompression of
a backup compressed on the server can be performed on the client
using pg_basebackup -Fp --compress server-lz4.

Dipesh Pandit, reviewed and tested by Jeevan Ladhe and Tushar Ahuja,
with a few corrections - and some documentation - by me.

Discussion: http://postgr.es/m/CAN1g5_FeDmiA9D8wdG2W6Lkq5CpubxOAqTmd2et9hsinTJtsMQ@mail.gmail.com