Mint: Pending payment check on startup should be non-blocking and periodic task

[callebtc]

We call await self._check_pending_proofs_and_melt_quotes() once during startup in ledger.py. This call is blocking but it doesn't need to be, it could run in the background.

We should also call this function periodically to make sure the mint cleans up any stuck payments that could have been caused due to communication errors with the Lightning backend. This can be solved using an endless task loop that repeats, say, once an hour.