mp911de / lettuce / 1141

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Add support for topology consensus #355

Previously, cluster topology refreshing could get stuck on a node that was previously discovered but got removed from the cluster. This was possible because multiple views were obtained and any arbitrary topology view was chosen.

Lettuce now implements two consensus algorithms: Healthy Majority and Known Majority. Healthy Majority is applied on the very first topology retrieval, Known Majority for all subsequent topology refreshes.

Healthy Majority votes for topology views containing the most nodes with healthy flags (without FAIL/PFAIL/NOADDR flags) to use a most healthy view. Known Majority selects topology views that contain nodes that were previously known. This consensus works for adding and removing nodes one-by-one or even multiple nodes. In case a cluster is split into even partitions the client can still get stuck on either side, but that issue can be solved by disabling dynamic refresh sources and specifying stable cluster seed nodes.

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