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.