Fix context deadline issues with live loader (#2501)

- Use exponential backoffs with propose txns, so if the system is flooded, we don't make it worse.
- Do not block if we find pending txns. Return err, and abort them in a goroutine.
- Do not reach a deadlock trying to remove posting lists from LRU cache. Do a best-effort removal.
- Decrease the number of times SyncIfDirty happens.