Add "warmup mode" for RPC server.

Start the RPC server before doing all the (expensive) startup
initialisations like loading the block index.  Until the node is ready,
return all calls immediately with a new error signalling "in warmup"
with an appropriate status message (similar to the init message).

This is useful for RPC clients to know that the server is there (e. g.,
they don't have to start it) but not yet available.  It is used in
Namecoin and Huntercoin already for some time, and there exists a UI
hooked onto the RPC interface that actively uses this to its advantage.

src/rpcserver.cpp

@@ -34,6 +34,10 @@ using namespace std;
 static std::string strRPCUserColonPass;
 
 static bool fRPCRunning = false;
+static bool fRPCInWarmup = true;
+static std::string rpcWarmupStatus("RPC server started");
+static CCriticalSection cs_rpcWarmup;
+
 //! These are created by StartRPCThreads, destroyed in StopRPCThreads
 static asio::io_service* rpc_io_service = NULL;
 static map<string, boost::shared_ptr<deadline_timer> > deadlineTimers;
@@ -744,6 +748,19 @@ bool IsRPCRunning()
     return fRPCRunning;
 }
 
+void SetRPCWarmupStatus(const std::string& newStatus)
+{
+    LOCK(cs_rpcWarmup);
+    rpcWarmupStatus = newStatus;
+}
+
+void SetRPCWarmupFinished()
+{
+    LOCK(cs_rpcWarmup);
+    assert(fRPCInWarmup);
+    fRPCInWarmup = false;
+}
+
 void RPCRunHandler(const boost::system::error_code& err, boost::function<void(void)> func)
 {
     if (!err)
@@ -870,6 +887,13 @@ static bool HTTPReq_JSONRPC(AcceptedConnection *conn,
         if (!read_string(strRequest, valRequest))
             throw JSONRPCError(RPC_PARSE_ERROR, "Parse error");
 
+        // Return immediately if in warmup
+        {
+            LOCK(cs_rpcWarmup);
+            if (fRPCInWarmup)
+                throw JSONRPCError(RPC_IN_WARMUP, rpcWarmupStatus);
+        }
+
         string strReply;
 
         // singleton request