All RPC module APIs except for `send_snapshot` should resolve as
soon as the message is sent, so these messages are passed via
`send_message_oneway_timeout`.
`send_snapshot` message is sent via `send_message_timeout` and
returns a `future<>`, which resolves when snapshot transfer
finishes or fails with an exception.
All necessary functions to wire the new Raft RPC verbs are also
provided (such as `register` and `unregister` handlers).
Signed-off-by: Pavel Solodovnikov <pa.solodovnikov@scylladb.com>
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