Turn mutation processing sequential

As calls to proposer.Propose happened from the routing a connection is
handled in they could happen in concurrently. This could have resulted
in Ops send to the store carrying an outdated (same or older than the
store) rev, which resulted in discarded ops and the store proceeded
while ignoring those. Which made the loop/store.Wait spin forever and
not return a store.Event to the caller means the txn was never
completed.

Described behaviour would lead to cascading failures where components
relying on the return of txns would block in other places and slowly
fail.