Design and implement the lockable election provider for multi-block election

[gpestana]

In the multi-block election context, the snapshot creation takes multiple blocks. The mutations on the ledgers must be frozen/buffered during the time while the snapshot creation is ongoing, to make sure the snapshot data remains consistent. The election provider must implement a trait LockableElectionDataProvider to signal when to set and release the lock in staking.

Currently, in #6034, the LockableElectionDataProvider implementation is a noop, which is OK to run the election in a single-page context.

Implement the freeze/buffering logic in the staking pallet when the data provider lock is set.