Prototype: Read-only ring state

[pr00se]

What this PR does:

This is a draft PR that adds a new READONLY InstanceState, and support to set the joinedState before starting the lifecycler.

• joinedState defaults to ACTIVE
• during startup, lifecycler will automatically join the ring in the joinedState
• only valid joinedStates are ACTIVE and READONLY
• allowed transitions:
  • ACTIVE -> READONLY
  • READONLY -> ACTIVE
  • PENDING -> READONLY
  • READONLY -> LEAVING
• augmented existing tests

Notes and/or TODOs:

• joinedState could (should?) be a config option instead, if we don't ever need to change it after creating the lifecycler
• setting joinedState after the lifecycler has joined the ring has no effect
  • i haven't tested, or done anything to prevent, setting it WHILE the lifecycler is joining (e.g. while it is waiting before joining, or observing tokens) -- what should happen here?
• if joinedState is set to READONLY but the lifecycler finds the instance in the ACTIVE state in the ring, it won't switch the ring state to match joinedState -- it probably should
• there are no existing tests for the cfg.ObservePeriod setting, but we should probably add some
• not sure if the TransferChunks state transitions are still necessary, and if we need to add JOINING -> READONLY as a valid one
• I didn't touch any of the actual operations or health checks yet -- e.g.: READONLY instances should be healthy for purposes of IsHealthy(), and used for Read operations.

Which issue(s) this PR fixes:

Fixes #

Checklist

• Tests updated
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]