etcd: add a read/write timeout to server

The default is for connections to last forever[1]. This leads to fds
leaking. I set the timeout so high by default so that watches don't have
to keep retrying but perhaps we should set it slower.

Tested on a cluster with lots of clients and it seems to have relieved
the problem.

[1] https://groups.google.com/forum/#!msg/golang-nuts/JFhGwh1q9xU/heh4J8pul3QJ
Merge pull request #880 from philips/add-http-server-timeouts

etcd: add a read/write timeout to server