Fix lock re-entrancy in process_append_entry_request_as_follower()

[penberg]

In my project, I have a struct that implements both the Cluster and
StateMachine traits, and, therefore, also share the same Mutex
that's passed to Replica.

However, process_append_entry_request_as_follower is holding on to the
state machine lock while attempting to acquire the cluster lock, which
results in re-entrancy that causes a deadlock (as reported by
no_deadlocks crate):

A reentrance has been attempted, but `std::sync`'s locks are not reentrant. This results in a deadlock. dependence cycle: [Thread(ThreadId(11)), Lock(20)]
Lock taken at:
   0: little_raft::replica::Replica<C,M,T,D>::process_append_entry_request_as_follower
             at /Users/penberg/src/database/little-raft/little_raft/src/replica.rs:709:33
   1: little_raft::replica::Replica<C,M,T,D>::process_message_as_follower
             at /Users/penberg/src/database/little-raft/little_raft/src/replica.rs:766:18
   2: little_raft::replica::Replica<C,M,T,D>::process_message
             at /Users/penberg/src/database/little-raft/little_raft/src/replica.rs:352:32
   3: little_raft::replica::Replica<C,M,T,D>::poll_as_follower
             at /Users/penberg/src/database/little-raft/little_raft/src/replica.rs:332:21
   4: little_raft::replica::Replica<C,M,T,D>::start
             at /Users/penberg/src/database/little-raft/little_raft/src/replica.rs:233:36

Reentrace at:
   0: little_raft::replica::Replica<C,M,T,D>::process_append_entry_request_as_follower
             at /Users/penberg/src/database/little-raft/little_raft/src/replica.rs:735:27
   1: little_raft::replica::Replica<C,M,T,D>::process_message_as_follower
             at /Users/penberg/src/database/little-raft/little_raft/src/replica.rs:766:18
   2: little_raft::replica::Replica<C,M,T,D>::process_message
             at /Users/penberg/src/database/little-raft/little_raft/src/replica.rs:352:32
   3: little_raft::replica::Replica<C,M,T,D>::poll_as_follower
             at /Users/penberg/src/database/little-raft/little_raft/src/replica.rs:332:21
   4: little_raft::replica::Replica<C,M,T,D>::start
             at /Users/penberg/src/database/little-raft/little_raft/src/replica.rs:233:36

Fix the deadlock by moving entry processing to a separate function,
which reduces the scope of the state machine lock so that it is no
longer held when we attempt to acquire the cluster lock.

Fixes #42.