Add support for running the controller with replicas>1

We need to ensure that the controller doesn't do the wrong thing with replicas>1, since that can happen during transient events like node/partition recovery, mis-configured upgrades, etc. Replicas>1 is a _desirable_ feature for some HA configurations.

Note that this is just about avoiding breakage. Horizontal scalability (active/active or sharding) is _not_ included in this issue.

Note also this could either be via a lock (as is typically done with other controllers), or via careful auditing of atomic operations and race recovery (sealed-secrets-controller only has very simple side-effects).

Start by locating the sealed-secrets-controller reconciliation entry points and reviewing how it behaves when replicas>1 during node or partition recovery. Compare a controller lock with auditing atomic operations and race recovery, then verify that transient multi-replica events do not cause incorrect side effects while preserving the stated non-goal of horizontal scalability.

Written by the indexing model from the issue text.