Support dynamic reloading of credentials config file

[jtackaberry]

User management, especially password rotation, is a common operational activity. Currently the credentials config file is only read once at startup, which means rotating a password requires a rolling restart of the cluster.

This FR is a minor quality-of-life improvement to support dynamic reloading of the user credentials config file and handle password changes, user additions, removals, and permissions changes.

This could be done using fsnotify, which would be fairly robust but requires sourcing a new dependency, or by simply using os.Stat() and polling for mtime changes every few seconds. Regardless of the method, the implementation should tolerate the path provided to rqlited being a symlink, and follow the symlink in detecting changes.