Delete completion in rambuffer

The fact we already acquired the new segment means that
the corresponding rambuffer is available.
Therefore, waiting for the rambuffer to complete is meaningless.
fix

Add line to wait for the RAM buffer to be avaliable

- wait_for_flushing and wait_for_migration now takes id
  as the 2nd argument
Merge branch 'feature/delete-buffer-completion' into develop
Add new test to check if migration half-way doesn't corrupt the
filesystem on the backing device