Fix prefix counters in the /status page and the /bmp-routers/ page

[density215]

All prefix counters everywhere remain at 0, even though store has all of them. Also prefixes per peer would be appreciated, this depends on NLnetLabs/rotonda-store#46

[DRiKE]

Fixed in in dev branch in bddb9c7

The stored_prefixes on the bmp-in unit level does not make sense anymore (as the rib units take care of all the storing now) so that has been removed from /status and the /bmp-routers page.
Note that we still have the num_received_prefixes, which counts the same as num_announcements. One of these will go, eventually.

For the rib, the num_uniqueue_prefixes was indeed not set properly, that should be fixed now.