Connection health monitoring

- Simplify node peers handling.
- Remove connection reference counting.
- Add connection health monitoring.
- Use monitoring output to only send back a healthy connection when asked to get a server or leader for a group.

Squashed commit of the following:

commit bb1f68a044d51f3f61d3d16b5a6ff7c6656e97e1
Author: Manish R Jain <manish@dgraph.io>
Date:   Thu Sep 14 17:42:25 2017 +1000

    No need to send export call to dgraphzero, it won't handle export anyway.

commit 0106decd948fc5f28242d9b098ca9cccbcbe31fc
Author: Manish R Jain <manish@dgraph.io>
Date:   Thu Sep 14 17:02:09 2017 +1000

    Tested that the connection switch automatically happens in favor of healthy one.

commit f4bfdb40ecf5f17a1cebc0f9d3f5249433d4c79c
Author: Manish R Jain <manish@dgraph.io>
Date:   Thu Sep 14 16:34:49 2017 +1000

    Add connection health checking. Also before returning connections to servers in a group, check health before returning.

commit 70ebcc44a5ab8731e10a027689a084cbe78885a2
Author: Manish R Jain <manish@dgraph.io>
Date:   Thu Sep 14 16:17:13 2017 +1000

    Remove refcounting. And start monitoring health of connection.