[chronik] index scripts by scripthashes in the mempool

Summary: So far we didn't have a script hash for new scripts that were never included in a block. This diff adds a mempool index using a HashMap.

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Subscribers: tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D17037