inject FromClient into correct buffer

This should fix ARAKOON-420 and ARAKOON-421.

The nodes die because they receive a FromClient message in the slave_wait_for_accept or wait_for_promises state.
A quick look at https://github.com/Incubaid/arakoon/blob/1.6/src/paxos/multi_paxos_fsm.ml#L739 and https://github.com/Incubaid/arakoon/blob/1.6/src/paxos/multi_paxos_fsm.ml#L732 shows that these states cannot receive those messages as they are not subscribed to the messages those buffers should be in.
The Nop inserted at https://github.com/Incubaid/arakoon/blob/1.6/src/paxos/master.ml#L43 however goes through the inject_event which is fixed in this pull request.