state/api/...: introduce and use common.Watch

Introduce and use a common Watch implementation,
which is the client-side equivalent to apiserver's
AgentEntityWatcher.
Merge pull request #125 from axw/state-api-common-watch

state/api/...: introduce and use common.Watch

Introduce and use a common Watch implementation,
which is the client-side equivalent to apiserver's
AgentEntityWatcher.