OSS portion of enterprise PR 1857.
This removes (most) references to the `cache.UpdateEvent` type in the
`proxycfg` package.
As we're going to be direct usage of the agent cache with interfaces that
can be satisfied by alternative server-local datasources, it doesn't make
sense to depend on this type everywhere anymore (particularly on the
`state.ch` channel).
We also plan to extract `proxycfg` out of Consul into a shared library in
the future, which would require removing this dependency.
Aside from a fairly rote find-and-replace, the main change is that the
`cache.Cache` and `health.Client` types now accept a callback function
parameter, rather than a `chan<- cache.UpdateEvents`. This allows us to
do the type conversion without running another goroutine.
The blocking query backend sets the default value on the server side.
The streaming backend does not using blocking queries, so we must set the timeout on
the client.
Dan Upton
Daniel Nephin