Implement automatic shutdown of deprecated Zcash versions

Closes #2274.

src/deprecation.h

@@ -0,0 +1,23 @@
+// Copyright (c) 2017 The Zcash developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef ZCASH_DEPRECATION_H
+#define ZCASH_DEPRECATION_H
+
+// Deprecation policy is 4th third-Tuesday after a release
+static const int APPROX_RELEASE_HEIGHT = 115000;
+static const int WEEKS_UNTIL_DEPRECATION = 18;
+static const int DEPRECATION_HEIGHT = APPROX_RELEASE_HEIGHT + (WEEKS_UNTIL_DEPRECATION * 7 * 24 * 24);
+
+// Number of blocks before deprecation to warn users
+static const int DEPRECATION_WARN_LIMIT = 14 * 24 * 24; // 2 weeks
+
+/**
+ * Checks whether the node is deprecated based on the current block height, and
+ * shuts down the node with an error if so (and deprecation is not disabled for
+ * the current client version).
+ */
+void EnforceNodeDeprecation(int nHeight, bool forceLogging=false);
+
+#endif // ZCASH_DEPRECATION_H