replication: reshuffle names of the state

In this patch will be introduced a new state for which the
name is suitable: `APPLIER_FETCH_SNAPSHOT`. But it's already
taken. The names of the state will be reshuffled a bit.

    `APPLIER_FETCH_SNAPSHOT -> APPLIER_WAIT_SNAPSHOT;`
    `APPLIER_INITIAL_JOIN -> APPLIER_WAIT_SNAPSHOT;`

Part of #6126

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NO_CHANGELOG=preparatory commit
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