[Chronik] Add MempoolTokens to index mempool token txs

Summary:
Parses, colors, validates and indexes mempool txs as SLP/ALP txs.

In fetch_tx_spent_tokens, bulk-query ops are used wherever possible. It is exposed so we can use it in the indexer later, too.

Test Plan: cargo test -p chronik-db test_mempool_tokens

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D15099