[electrum] bump RPC timeout

Summary:
For addresses with a very large history, with a low-end server, the blockchain.scripthash.subscribe and blockchain.scripthash.get_history RPCs can take longer than 10 seconds.

Also use constants instead of magic numbers for timeouts.

Test Plan: Start syncing a very large wallet in Electrum ABC on a weak server, check that it manages to fetch the history

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18651