Don't spuriously time out restore sessions

If a restore set lookup took a long time, the client's restore
session could be declared timed out even though the client was
not at fault.  Handle this properly by resetting the timeout clock
when control of the session is returned to the client.

Bug 3477324

Change-Id: I43afaf1063e8e706ef16b70be77f9eeeea6a321f