Search for HTLC related histories via preimage hash and/or HTLC ID

[abitmore]

Once an HTLC is redeemed or expired in BitShares blockchain, it's hard to search for related activities via API.

Use story: as the other party of a HTLC I want an easy way to query for the preimage so I can redeem it in another blockchain.

Perhaps need to add something to ES plugin?