Cap the number of simultaneous node replacements

[mflendrich]

If Operator decides to replace a bunch of Scylla nodes, there is a possibility that replacement will bring many (even all) of them down, causing loss of quorum.

Acceptance Criteria

• Operator imposes a cap on the number of simultaneous replacements as configured
• It has to support "uncapped" and "1 at a time". (At the time of writing I don't think we need a "N>1 at a time" mode.

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