Introduce Mining interface

Start out with a single method isTestChain() that's used by the getblocktemplate RPC.

src/node/context.cpp

@@ -7,6 +7,7 @@
 #include <addrman.h>
 #include <banman.h>
 #include <interfaces/chain.h>
+#include <interfaces/mining.h>
 #include <kernel/context.h>
 #include <key.h>
 #include <net.h>