Fix claimpegins from mempool locks

[instagibbs]

This PR also includes a change which changes free relay to only work for withdrawlock spends, since those cannot include a fee, and removes the rate limiting. Pegging transactions are already rate-limited by Bitcoin fees on the mainchain, and miners will select higher-fee transactions, so DoS is unlikely.

[jonasnick]

I've created a script that creates and confirms n_locks withdrawlocks and attempts to claim n_claims in mempool: https://gist.github.com/jonasnick/ecea5fcf8d2be5ef285cee7e40a69177.

n_locks = 100, n_claims = 110: works
n_locks = 100, n_claims = 200: Failed to merge locked outputs, please try again later (258: txn-mempool-conflict)
n_locks = 500, n_claims = 500: Failed to merge locked outputs, please try again later (16: mandatory-script-verify-flag-failed (Script is too big))

I would have expected that the second case would have resulted in a too-long-chain error but not that a claim would try to respend a withdrawlock.

[instagibbs]

@jonasnick Pushed fixes and poached your flooding test.

[kallewoof]

utACK 793e92b