Add ReshardingOperation::Start consensus message

[ffuugoo]

Tracked in #4213.

This PR adds ReshardingOperation::Start consensus message. And also (in the most trivial way) disables splitting read operations to the resharding shard.

Important TODO to be fixed in some future PR: consensus snapshot does not handle resharding in any way yet.

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[timvisee]

Eventually we probably want to extend this to allow more shard configurations, but we can do that then.