Feature: (configurably) tell the user who's currently using the board

[konradybcio]

Sometimes you can see

Starting cdba server
cdba-server: board is in use, waiting...

which doesn't help you to determine e.g. whether the board/connection has hung with no timeout or someone is actually using the board, and if so, who that is.. Add an option (for obvious reasons) to demistify this a bit.. Maybe even something like this:

Starting cdba server
cdba-server: board is currently in use by konrad (1337 seconds), waiting...