ReplicatedBackend: don't hold ObjectContexts in pull completion callback

ReplicatedBackend: don't hold ObjectContexts in pull completion callback

We need flushing the sequencer to ensure that all Contexts which hold
ObjectContextRefs have been run or deleted.
C_ReplicatedBackend_OnPullComplete, however, gets queued in a second
work queue in order to avoid performing expensive push related reads
in the FileStore finisher.

Rather than keep the objects contexts around, we instead put off
removing the object from the pulling map until the call back
fires and read the object context out of the pulling map.  This
way the ObjectContextRef will be cleaned up along with the rest
of the pulling map in on_change.

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