srm: Restore estimated wait time update

Commit 5211277 removed some "dead wood" (as the patch description called
it), but it turns out that some of the wood wasn't quite dead - just old.

Since we all grow older, this patch brings justice to the old trees and
restores the polling backoff that used to be part of the SRM. Clients will
once again reduce the polling frequency for requests that seem to take
a long time.

Target: trunk
Request: 2.10
Request: 2.9
Require-notes: yes
Require-book: no
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: https://rb.dcache.org/r/7175/