Fix possible race condition for accept transfer

Accept transfer API workflow is currently like this:

  call volume_api.accept_transfer()
    |
    --- RPC cast to volume manager
          |
          --- volume manager calls volume driver accept_transfer()

  update volume's DB record

Given the non-blocking nature of RPC cast, what happens in volume
manager and volume driver can happen in parallel with the DB update.
If volume driver relies on original DB record to do things, then
DB record shouldn't be updated until volume driver finishes its job.

So this patch change volume RPC API accept_transfer() from cast
to call to make sure the workflow is in serialized manner.  Also
elevated the context when volume manager tries to update the DB
record when driver has done accept_transfer().

Change-Id: Ieae52e167aa02967338e0be5d78d570d682faa7a
Closes-bug: #1357432
Merge "Fix possible race condition for accept transfer"