[rpc] increase the offset of rpc port

The 10/20 offset will upset local test with more than 10 nodes being
launched.

The error I encountered is like:

panic: failed to listen on any addresses: [listen tcp4 0.0.0.0:9015: bind: address already in use]

goroutine 1 [running]:
github.com/harmony-one/harmony/p2p/host/hostv2.catchError(...)
        /Users/leochen/work/go/src/github.com/harmony-one/harmony/p2p/host/hostv2/util.go:10

Signed-off-by: Leo Chen <leo@harmony.one>

node/rpc.go

@@ -13,8 +13,8 @@ import (
 )
 
 const (
-	rpcHTTPPortOffset = 10
+	rpcHTTPPortOffset = 500
-	rpcWSPortOffset   = 20
+	rpcWSPortOffset   = 800
 )
 
 var (