Replaces State() (which returned ACTIVE/STARTED/etc) with IsActiveAfter()
which just returns a bool, as this was all State was actually used
for. Drops Mask(), which was only used in tests and can be replaced with
`1<<bit`, and also drops StateSinceHeight() and Statistics(), which are
now only used internally for Info().
Adds support for versionbits deployments to DeploymentEnabled,
DeploymentActiveAfter and DeploymentActiveAt. Also moves versionbitscache
from validation to deploymentstatus.
Provides DeploymentEnabled, DeploymentActiveAt, and DeploymentActiveAfter
helpers for checking the status of buried deployments. Can be overloaded
so the same syntax works for non-buried deployments, allowing future
soft forks to be changed from signalled to buried deployments without
having to touch the implementation code.
Replaces IsWitnessEnabled and IsScriptWitnessEnabled.