Calculate blockchain_hex scale only using active hotspots

[vihu]

Some community members have reported that the hex scale calculation factors in known "offline" hotspots (those which haven't had any POC activity for a while dependent on challenge interval variable).

Although this was done in initial design as the "scale" value of a hotspot solely depends on the density of hotspots in that region, however, as reported this causes honest, well-performing hotspots to suffer lower scale values by being in the same region as offline hotspots.

Notes:

• Change blockchain_hex module to factor in online/offline hotspots (also do this so it doesn't take too much time, more discussion probably required how to).
• We only use blockchain_hex:scale when calculating rewards in core not in any poc related transactions, presumably if blockchain_hex:scale returns "correct" scale we don't need to change anything there.