PG: start flush on primary only after we process the master log

Once we start serving reads, stray objects must have already
been removed.  Therefore, we have to flush all operations
up to the transaction writing out the authoritative log.
On replicas, we flush in Stray() if we will not eventually
be activated and in ReplicaActive if we are in the acting
set.  This way a replica won't serve a replica read until
the store is consistent.

Signed-off-by: Samuel Just <sam.just@inktank.com>