rpc: Split option -rpctimeout into -rpcservertimeout and -rpcclienttimeout

The two timeouts for the server and client, are essentially different:

- In the case of the server it should be a lower value to avoid clients
clogging up connection slots

- In the case of the client it should be a high value to accomedate slow
  responses from the server, for example for slow queries or when the
  lock is contended

Split the options into `-rpcservertimeout` and `-rpcclienttimeout` with
respective defaults of 30 and 900.

src/httpserver.cpp

@@ -374,7 +374,7 @@ bool InitHTTPServer()
         return false;
     }
 
-    evhttp_set_timeout(http, GetArg("-rpctimeout", DEFAULT_HTTP_TIMEOUT));
+    evhttp_set_timeout(http, GetArg("-rpcservertimeout", DEFAULT_HTTP_SERVER_TIMEOUT));
     evhttp_set_max_body_size(http, MAX_SIZE);
     evhttp_set_gencb(http, http_request_cb, NULL);