[Version-unspecific] Fix ZEC pending UTXO reservations

Summary

Production ZEC outbounds were missing their "pending UTXO" entries whenever a signer crashed or retried in a new height. Because the reservation key used to be the raw txid (only written after broadcast), errors during signing or network outages left no record, so the next attempt reselected identical inputs and got stuck behind “no utxo to spend found”. In a some cases we even rebuilt a fresh transaction for a still‑pending outbound and double‑spent ourselves.

This MR:

• removes the Zcash signer’s in-memory reservation cache so every pending height lives in LevelDB, making commit/cleanup code much simpler
• updates the LevelDB helpers to return the stored height alongside the UTXO list, and adjusts the signer to log/clean reservations based on that single lookup