net.server: Add a predrain callaback just before writes

Allows sneaking in things in the write buffer just before it's sent to
the network stack. For example ack requests, compression flushes or
other things that make sense to send after stanzas or other things.
This ensures any additional trailing data sent is included in the same
write, and possibly the same TCP packet. Other methods used such as
timers or nextTick might not have the same effect as it depends on
scheduling.

net/server_epoll.lua

@@ -472,6 +472,7 @@ end
 function interface:onwritable()
 	self:onconnect();
 	if not self.conn then return; end -- could have been closed in onconnect
+	self:on("predrain");
 	local buffer = self.writebuffer;
 	local data = buffer or "";
 	if type(buffer) == "table" then