patch to allow disable of WAL recycling

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.

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