Consider adding a service reload API endpoint

[brendan-ward]

As mentioned in #128

We may want to consider adding a reload API endpoint to trigger reload of a specific service or set of services, rather than restarting the entire server process. We'd probably need to set a mutex (so multiple calls to /reload in separate threads don't collide), then uses something similar to what we're doing in the filesystem watcher to rescan the registered directories and re-add those that exist, or take a service ID and reload just that one.

We need to figure out how to handle auth for this; some options:

• shared secret (your token); which is nice and lightweight but also risky, it could get logged as plaintext anywhere in between caller and mbtileserver and potentially used to do a DOS attack
• HTTP Basic Auth based on credentials provided at server startup
• something else?