Add a new module to watch host-file

[BinChang]

To use tChannel in a peer-to-peer fashion, without hyperbanh, we usually store server nodes in a host-file. A host-file watcher like (uber/tchannel-node#325) will things much easier. Several services in Uber (especially in marketplace) could benefit from this.

This is sort of urgent, I could draft a PR if you guys are all busy with more important stuff.

[prashantv]

@BinChang would be great if you could draft a PR for this. The core TChannel library provides this PeerList object which lets others manage peers:
https://godoc.org/github.com/uber/tchannel-go#Channel.Peers

I think file watching logic should not live inside of the core tchannel-go package, but in a separate subpackage. I would expect some API like:

import "github.com/uber/tchannel-go/peermanager"

// assume we have an existing channel, ch
hostWatcher, err := peermanager.NewFileSyncer(ch.Peers(), "hosts.json")

This would then spin up a goroutine that would watch the hosts file, and manage calls to Peers.Add and Peers.Remove when the file has changed.

Let me know if you have any questions. I'll send you a link to some internal peer updaters we have as well.

[BinChang]

SGTM.