[refactor] Add deploymentstatus.h

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.

src/init.cpp

@@ -16,6 +16,7 @@
 #include <chain.h>
 #include <chainparams.h>
 #include <compat/sanity.h>
+#include <deploymentstatus.h>
 #include <fs.h>
 #include <hash.h>
 #include <httprpc.h>
@@ -1587,7 +1588,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
         }
     }
 
-    if (chainparams.GetConsensus().SegwitHeight != std::numeric_limits<int>::max()) {
+    if (DeploymentEnabled(chainparams.GetConsensus(), Consensus::DEPLOYMENT_SEGWIT)) {
         // Advertise witness capabilities.
         // The option to not set NODE_WITNESS is only used in the tests and should be removed.
         nLocalServices = ServiceFlags(nLocalServices | NODE_WITNESS);