handle lock time expire before swapConf reached

[chappjc]

If the lock time on a contract expires before the swap txn reaches swapConf (or even gets mined), special handling is needed. This should be quite rare to have a txn with the required mining fees to sit in mempool for 8 or more hours, but it could happen.

Server:

• revoke the match without penalty to either party
• maker's contract should cause a revoke even sooner than expiration if the lock time remaining is less than 12 hrs, leaving insufficient time for taker's contract.

Client:

• Maker's swap txn unmined/immature
  • On expire, maker should refund. A revoke_match may arrive, but it is not to be required. This is 20hrs so should be extremely unlikely.
  • Even when the maker's contract is mined with swapConf confirms, taker needs to always check that the maker's contract has at least 12 hrs (or 8?) remaining until expiry before broadcasting their own contract. (separate issue?)
• Taker's swap txn unmined/immature: Taker should refund. Maker should under no circumstances redeem the contract (thus revealing the secret for the counter party to use in their redeem and attempt a refund double spend). If the maker does redeem, taker should extract the secret and redeem the other contract.

[chappjc]

Resolved by: https://github.com/decred/dcrdex/pull/1866 and https://github.com/decred/dcrdex/pull/1548 and https://github.com/decred/dcrdex/pull/1541 and others.