Vault aborting on Wallet already loading. error

[sander2]

Related: #381

When starting a vault while the bitcoin node is still syncing, I got this behavior:

Dec 20 11:12:44.359  INFO bitcoin: Waiting for bitcoin-core to sync...    
Dec 20 11:26:32.005  INFO bitcoin: Synced!    // NOTE: this took a long time
Dec 20 11:26:32.029  INFO bitcoin: Loading wallet vault-live-polkadot-master...    
Dec 20 11:26:47.234  INFO bitcoin: Call timed out - retrying...    
Dec 20 11:26:47.235  INFO bitcoin: Loading wallet vault-live-polkadot-master...    
Dec 20 11:26:47.265  INFO vault::process: Removing PID file at: /tmp/interlay-parachain_5HCEA2NynWi1J7xfup8dvjV6tVYGN5GkVYMLhddTn8ujFTur.pid
Dec 20 11:26:47.273 ERROR vault: Exiting: BitcoinError: BitcoinError: JSON-RPC error: RPC error response: RpcError { code: -4, message: "Wallet already loading.", data: None }