Since users are connecting to their own full node, there's little value in having Electrum verify the SPV proofs, and we can save some resources by not generating them. To be used with Electrum's --skipmerklecheck.

• Implement --electrum-skip-merkle (2390f2d)
• Auto-enable --skipmerklecheck and --electrum-skip-merkle in the Electrum plugin (e6a3399)