Use input amount and type data for metrics

[0xB10C]

The prevout data is available in the backend since #27, but not really used yet. It can be used for:

• UTXO set by value (see New chart request: UTXO set composition over time #14)
• fee and feerate metrics and charts
• spent coin age charts (Chart Request: What percentage of UTXOs are spent from the prev block / prev 6 blocks / prev 144 / prev 2016 blocks #31)
• new metric / chart: unnecessary input heuristic (UIH) #66

[darosior]

• fee and feerate metrics and charts

For what it's worth i came looking for this again in the context of the sub-1sat/vb discussions. I wanted to see if it consisted a meaningful share of the traffic.

[0xB10C]

#89 partly addresses the fee and feerate metrics chart part. Also adds a chart for sub-1sat/vb tx.

[0xB10C]

Primary goal of merged #89 was sub-1sat/vb tx stats. Some followups for feerate bands, package feerate percentiles and vsize percentiles in #89 (comment).