raft: apply snapshots in applier_fiber
raft: apply snapshots in applier_fiber

We want to serialize snapshot application with command application
otherwise a command may be applied after a snapshot that already contains
the result of its application (it is not necessary a problem since the
raft by itself does not guaranty apply-once semantics, but better to
prevent it when possible). This also moves all interactions with user's
state machine into one place.

Message-Id: <88b914b051849764e60a106f57b797e33f174450@scylladb.com>

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