feat: add WatchChan method.

[zbindenren]

WatchChan creates a watcher for a key or prefix and unmarshals events into channel. The channel elements have to implement the store.KeyOpSetter interface.

[coveralls]

Pull Request Test Coverage Report for Build 1630872576

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 145 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-1.6%) to 81.079%

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Files with Coverage Reduction	New Missed Lines	%
hash/hash.go	6	95.42%
etcd/etcdv3.go	12	87.62%
hash/watch.go	12	86.36%
store.go	14	33.33%
etcd/watch.go	34	76.92%
etcd/config.go	67	60.67%

Totals
Change from base Build 1448118989:	-1.6%
Covered Lines:	1037
Relevant Lines:	1279

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💛 - Coveralls