Keep index synced in background

[casey]

Currently, the index syncs whenever you run a command. It would be nice if it were possible to run the index as a daemon in the background, and have ord connect to it whenever it needed to query the index state. This is a pretty large feature, since we'd need to:

• Add a command for running the index as a daemon
• Create an API for querying the daemon
• Modify existing commands to communicate with the daemon via the API
• Figure out if this mode is required or optional, e.g. can you still use ord without having ord daemon run in the background?