* Remove obsolete header cache
why:
Was fall back for the case that the DB table was inaccessible before
`FC` module reorg.
* Add the number of unused connected peers to metric
* Update docu, add Grafana example
why:
Provides useful settings, e.g. for memory debugging
* Re-calibrate blocks queue for import
why:
Old queue setup provided a staging area which was much too large
consuming too much idle memory. Also the command-line re-calibrating
for debugging was much too complicated.
And the naming for the old setup was wrong: There is no max queue
size. Rather there is a HWM where filling the queue stops when reached.
The currently tested size allows for 1.5k blocks on the queue.
* Rename hidden command-line option for debug/re-calibrating blocks queue
