region/client: send request timeout to HBase

TODO:
- how to write a test for this?
- what exception is returned?
- are we guaranteed to get a reply if the request timeout?
- what if the requests is dropped from the queue?

region/client.go

@@ -626,6 +626,12 @@ func (c *client) send(rpc hrpc.Call) (uint32, error) {
 		RequestParam: proto.Bool(true),
 	}
 
+	deadline, ok := rpc.Context().Deadline()
+	if ok {
+		timeout := deadline.Sub(time.Now()).Abs()
+		header.Timeout = proto.Uint32(uint32(timeout.Milliseconds()))
+	}
+
 	if s, ok := rpc.(canSerializeCellBlocks); ok && s.CellBlocksEnabled() {
 		// request can be serialized to cellblocks
 		request, cellblocks, cellblocksLen = s.SerializeCellBlocks()