There are two variants one with storage and identify support and the
other without storage.

It implements the following commands:
 * INSERT id
 * REMOVE id
 * SCAN id
 * ERROR error-code
 * LIST (returns saved print)

The INSERT/REMOVE/LIST commands are only available in the storage
driver. The SCAN command emulates presenting a finger.

These commands can be send ahead of time, and will be queued and
processed when appropriate. i.e. for INSERT/REMOVE that is immediately
when possible, for SCAN/ERROR processing is delayed.

The LIST command is always processed immediately.

Note that only a single command can be send per socket connection and
the command must be send in a single message. The socket will be closed
after the command has been processed.

Co-authored-by: Bastien Nocera <hadess@hadess.net>
Co-authored-by: Marco Trevisan (Treviño) <mail@3v1n0.net>