HTTP watch merging

Having a HTTP request per watch is expensive, if you have a lot of watches. So would be nice to be able to set multiple watches with single request in API v2. As an alternative zookeeper-like protocol where you set watches asynchronously would be ok too (but that probably would mean not a HTTP API which has it's own downsides).

The exact cost is actually not a TCP connection per request (localhost connections are not very expensive). But because HTTP doesn't have a way to "ping" other peer (unlike e.g. zookeeper protocol), you need to restart every request each couple of seconds just to be sure nothing is hanging.