Cache the total points tracked in our historical liquidity data

Cache the total points tracked in our historical liquidity data

When we go to score a channel using the historical liquidity data,
the first thing we do is step through all the valid bucket
combinations, multiply the min and max bucket, and then add them
together to calculate the total number of points tracked. This
isn't a free operation, and for scorers without much data it
represents a large part of the total time spent scoring during
routefinding.

Thus, here we cache this value, updating it every time the buckets
are updated.