osdc: less aggressive prefetch in read/write Journaler

osdc: less aggressive prefetch in read/write Journaler

Previously, if doing a write/is_readable/write/is_readable sequence,
you'd end up doing a flush after every write, even though there
was already a flush in flight that would advance the readable-ness
of the journal.

Because this flush-during-read path is only active when using
a read/write journal such as in PurgeQueue, tweak the behaviour
to suit this case.

Signed-off-by: John Spray <john.spray@redhat.com>