Move pMiningKey init out of StartRPCThreads

This commit decouples the pMiningKey initialization and shutdown from the RPC
threads.

`getwork` and `getblocktemplate` rely on pMiningKey, and can also be ran
from the debug window in the UI even when the RPC server is not running.

Solves issue #2706.

src/rpcmining.cpp

@@ -11,6 +11,21 @@
 using namespace json_spirit;
 using namespace std;
 
+// Key used by getwork/getblocktemplate miners.
+// Allocated in InitRPCMining, free'd in ShutdownRPCMining
+static CReserveKey* pMiningKey = NULL;
+
+void InitRPCMining()
+{
+    // getwork/getblocktemplate mining rewards paid here:
+    pMiningKey = new CReserveKey(pwalletMain);
+}
+
+void ShutdownRPCMining()
+{
+    delete pMiningKey; pMiningKey = NULL;
+}
+
 Value getgenerate(const Array& params, bool fHelp)
 {
     if (fHelp || params.size() != 0)