Occur panic if nodes lose connections to redis

[rantan]

This occurred on Testnet like this.

[2021-09-17T00:56:17Z ERROR tapyrus_signer::signer_node] Connection Manager Error ConnectionManagerError { description: "Connection reset by peer (os error 104)", cause: Some(Connection reset by peer (os error 104)) }
thread 'main' panicked at 'ConnectionManagerError { description: "Connection reset by peer (os error 104)", cause: Some(Connection reset by peer (os error 104)) }', src/signer_node/mod.rs:230:25
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

I think the node should try to re-connect if it loses a connection to Redis until get back the connection.