Server needs to report disk free in bitcoin data directory

[proehlen]

With an ever increasing block size it would be useful to monitor the the amount of space available in the bitcoin data directory so preemptive action can be taken before it is full.

To this end, we need:

• a new GraphQL end point for this and related information.
• possibly consider adding it to the info broadcast by the websocket but only poll the available space every X minutes rather than ever second.

On linux, the command would be df -h <bitcoin data folder>. We also need to cater to windows and maybe mac (the latter would be for developer machines mostly but the query should still work).