Add cntl string to modify retry backoff

Intentionally left undocumented

Change-Id: I04606de2e421de11922edde6fec4c2a85f9d1c8a
Reviewed-on: http://review.couchbase.org/41979
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>