Introduce Mining interface

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

src/interfaces/init.h

@@ -7,6 +7,7 @@
 
 #include <interfaces/chain.h>
 #include <interfaces/echo.h>
+#include <interfaces/mining.h>
 #include <interfaces/node.h>
 #include <interfaces/wallet.h>
 
@@ -32,6 +33,7 @@ public:
     virtual ~Init() = default;
     virtual std::unique_ptr<Node> makeNode() { return nullptr; }
     virtual std::unique_ptr<Chain> makeChain() { return nullptr; }
+    virtual std::unique_ptr<Mining> makeMining() { return nullptr; }
     virtual std::unique_ptr<WalletLoader> makeWalletLoader(Chain& chain) { return nullptr; }
     virtual std::unique_ptr<Echo> makeEcho() { return nullptr; }
     virtual Ipc* ipc() { return nullptr; }