Add a tx watcher in Lua

[glihm]

Sometimes, users may not want to user the watch_interval. Having a Tx watcher can make them configure how they want the waiting.

Also, we can accept an array-like, in order to watch several txs like a join for threading.

watch_tx("tx_hash", interval_ms);

watch_txs({ "tx_hash1", "tx_hash2" }, interval_ms);

• Single watcher
• Multi watcher

[glihm]

Single watcher added in 2c50f6d.