Replication slots are useful to retain data that may be needed by a
replication system.  But experience has shown that allowing them to
retain excessive data can lead to the primary failing because of running
out of space.  This new feature allows the user to configure a maximum
amount of space to be reserved using the new option
max_slot_wal_keep_size.  Slots that overrun that space are invalidated
at checkpoint time, enabling the storage to be released.

Author: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Jehan-Guillaume de Rorthais <jgdr@dalibo.com>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20170228.122736.123383594.horiguchi.kyotaro@lab.ntt.co.jp