Add logs for cluster major events

[adiholden]

No description provided.

[chakaz]

To give some more context here:

Yesterday we had a brainstorming session in which we discussed what needs to be logged to help future debugging / investigations. Specifically we wanted to make sure that dangerous / important events are covered well by logs.

We compiled the below list. Note that some logs already exist, so we only need to add logs (including important parameters) where missing.

• Connection breakdown (migration / replication)
• Print all cluster config pushes, with an option to show diff from previously set config (like new nodes, new slots, migration, etc)
• Migration related events: rollback, stop / cancel, finalization
• flush slots that is done not due to successfully finalizing a migration (like in the case of config that cancels a migration)
• All DFLYCLUSTER commands with their arguments
• Apply config which reverts an already finalized migration, causing a data loss on the target node

[chakaz]

As part of this issue, we should also include cluster .cc files in default --vmodule