Epic: safekeepers dynamic membership change (Phase 1)

[arssher]

We need to be able to move tenants between safekeepers:

1. To migrate all tenants on one safekeeper if it dies.
2. To migrate one tenant if we find that load balancing needs it.

Eventually a proper dynamic membership change two step algorithm should be implemented (), which works correctly regardless of old safekeeper(s) state. However this is complicated, so let's do it in following large steps:

Tasks

Beta Give feedback

1. Revive procedure for moving tenants manually between safekeepers #4783
   c/storage/safekeeper +1
   
2. Make pull_timeline production-ready #6340
   c/storage/safekeeper +1
   
3. Still rely on physical old node stop/pause, but move implementation to compute (and console) instead of manual pull_timeline call, providing infra for the next step. TBD, but roughly this is an endpoint in console which for timeline in tenant 1) queries compute until it makes semantically sync-safekeepers 2) updates safekeepers in the db 3) updates config on compute(s).

[stepashka]

we aim to work on this this year