[Bug] SIGTERM/SIGINT during a mempool-update RPC reconnect logs a warning but electrs keeps running indefinitely #164

reported by @zenul_abidin:matrix.org

I had GPT-6 Astra do a static analysis which it confirmed in local v3.3.0 and mempool commit eda20be with the following summary:

src/signal.rs: Waiter consumes the signal and returns ErrorKind::Interrupt; no persistent shutdown state exists.
src/daemon.rs: reconnect waits propagate that error.
src/new_index/mempool.rs: Mempool::update() wraps it with chain_err().
src/bin/electrs.rs: both initial and periodic mempool-update handlers log and discard all errors, losing the shutdown request.