crimson/os/seastore: mkfs_managers() into coroutines

crimson/os/seastore: mkfs_managers() into coroutines

* txn is now created only once and not in each repeated
  iteration as before, with_repeat_trans_intr will reset the txn
  if needed.

* in order for us to return a seastar::future, we would have to handle
  each and every errorator. Instead, change the return value to
  errorator and let the caller handle the error.

* The lambda which is passed to with_trans_intr returns mkfs_iertr
  since this is an extended version of base_iertr.
  Introduce a mkfs_ertr for the reason above. Future commits would
  unify the new mkfs_ertr with seastore_mkfs_ertr.

Signed-off-by: Matan Breizman <mbreizma@redhat.com>