Add functionality to end airdrop

[0xean]

Background

Once the airdrop has been completed, we should transfer all tokens back to the reserve and prune all records from the store.

This should be added to the EndBlocker and check if the airdrop time is over.

Some good examples of how this should work:

https://github.com/osmosis-labs/osmosis/blob/v4.2.0/x/claim/abci.go
and
https://github.com/osmosis-labs/osmosis/blob/e79fc9f47f2bd4518d9e16567a369c25a0820fa6/x/claim/keeper/claim.go#L31

Acceptance Criteria

• endblocker checks for airdrop expiration
• once airdrop is over it transfers unclaimed tokens to reserve
• tests added to show this functionality working correctly.