patch to allow disable of WAL recycling

Hello All,

Attached is a patch to provide an option to disable WAL recycling. We have
found that this can help performance by eliminating read-modify-write
behavior on old WAL files that are no longer resident in the filesystem
cache. The is a lot more detail on the background of the motivation for
this in the following thread.

https://www.postgresql.org/message-id/flat/CACukRjO7DJvub8e2AijOayj8BfKK3XXBTwu3KKARiTr67M3E3w%40mail(dot)gmail(dot)com#CACukRjO7DJvub8e2AijOayj8BfKK3XXBTwu3KKARiTr67M3E3w(at)mail(dot)gmail(dot)com

A similar change has been tested against our 9.6 branch that we're
currently running, but the attached patch is against master.

Thanks,
Jerry