Collect the raw mempool txids into a HashSet

[tankyleo]

    Collect the raw mempool txids into a HashSet

    Mempool reconciliation checks every cached transaction against the raw
    mempool response. Using a Vec to store the raw mempool txids makes each
    membership check linear and the retention pass quadratic. Store the
    txids in a HashSet to make membership checks expected constant time and
    reconciliation expected linear.

    Co-Authored-By: HAL 9000

[ldk-reviews-bot]

👋 Thanks for assigning @benthecarman as a reviewer!
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