Fix server_state FSM transitions

[ayurchen]

There are two mutually dependent issues with wsrep::server_state FSM transitions:

1. it prematurely switches to s_synced on donor when SST start fails:
   https://github.com/codership/wsrep-lib/blob/3f79d4390ce4021da178b458633b430492fd3bf1/src/server_state.cpp#L672
2. it arbitrarily switches to s_joined, since there is no dedicated JOINED callback/event in the API:
   https://github.com/codership/wsrep-lib/blob/3f79d4390ce4021da178b458633b430492fd3bf1/src/server_state.cpp#L722

However this all can be resolved if we consider that s_joined state is actually reached when the last committed seqno becomes greater or equal to connected seqno.