Invalid Preimage Check/Test

[TheBlueMatt]

From #279 (comment), we should check that a transaction at 1 confirmation that is apparently an HTLC-Success claim but has the wrong preimage results in no action (and we're still willing to fail-back the HTLC after HTLC_FAIL_ANTI_REORG_DELAY confirmations if we don't learn a preimage). I believe this may be broken currently.