Support zebrad (Zcash Rust node) — skip ZMQ when message_queue_binding is empty

[tuan-phan]

Problem

Blockbook crashes when used with zebrad (the Rust-based Zcash node that replaces zcashd) because zebrad does not implement ZMQ.

After completing initial sync, blockbook calls InitializeMempool which unconditionally attempts to connect to ZMQ via NewMQ(). When message_queue_binding is empty (or zebrad is not listening on the ZMQ port), the call fails with EINVAL and blockbook exits fatally:

E0408 12:03:42 bitcoinrpc.go:207] mq: invalid argument
E0408 12:03:42 blockbook.go:340] initializeMempool invalid argument

Context

• zcashd (C++ fork of bitcoind) supported ZMQ, but it is deprecated
• zebrad is the actively maintained Zcash node and does not implement ZMQ
• Blockbook's mempool polling via getrawmempool RPC works fine with zebrad — the issue is only with the mandatory ZMQ initialization

Suggested Fix

In bchain/coins/btc/bitcoinrpc.go, InitializeMempool should skip ZMQ when MessageQueueBinding is empty:

// Current: always attempts ZMQ
if b.mq == nil {
    mq, err := bchain.NewMQ(b.ChainConfig.MessageQueueBinding, b.pushHandler, bitcoinTopics)

// Proposed: skip ZMQ when binding is empty, rely on RPC polling
if b.mq == nil && b.ChainConfig.MessageQueueBinding != "" {
    mq, err := bchain.NewMQ(b.ChainConfig.MessageQueueBinding, b.pushHandler, bitcoinTopics)

This is backward-compatible — all existing coins with real ZMQ bindings are unaffected. Mempool continues to work via the periodic getrawmempool polling (-resyncmempoolperiod).

An alternative approach would be to add a ZEC-specific override in bchain/coins/zec/zcashrpc.go.