This commit moves the subsidy halving interval to the chain params so it
can be configured per network. With that change it sets the regression
test halving interval to 150 to match the regression test params of the
reference implementation.
This was pointed out by @flammit.
Rather than converting the proof of work limit to its compact
representation multiple times during operation, do it once at package
initialization time and export it via the chain parameters.
This commit modifies the code to use params based on the active network
which paves the way for supporting the special rules and different genesis
blocks used by the test networks.
