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/bitcoinrpc.h

@@ -123,7 +123,9 @@ public:
 };
 
 extern const CRPCTable tableRPC;
-extern CReserveKey* pMiningKey;
+
+extern void InitRPCMining();
+extern void ShutdownRPCMining();
 
 extern int64 nWalletUnlockTime;
 extern int64 AmountFromValue(const json_spirit::Value& value);