Throttle Electrum requests so scans don't overwhelm public servers

The Electrum data path fired every request at once, so a scan burst dozens of
calls at a rate-limited public server, which then dropped some (the timeouts).
Add a concurrency cap (4 in flight; Electrum pipelines, so it stays fast) with
excess queued — mirrors the Esplora scheduler's intent. Verified a full scan via
the public jochen server completes in ~4.5s with peak concurrency 4, no timeouts.
The throttle queue is drained on disconnect/stop alongside in-flight calls.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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