Co-routinize paxos_state functions to make them more readable.
* 'gleb/coroutineze-paxos-state' of github.com:scylladb/scylla-dev:
paxos: simplify paxos_state::prepare code to not work with raw futures
paxos: co-routinize paxos_state::learn function
paxos: remove no longer used with_locked_key functions
paxos: co-routinize paxos_state::accept function
paxos: co-routinize paxos_state::prepare function
paxos: introduce get_replica_lock() function to take RAII guard for local paxos table access