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.
This commit updates mesh gateway watches for cross-partitions
communication.
* Mesh gateways are keyed by partition and datacenter.
* Mesh gateways will now watch gateways in partitions that export
services to their partition.
* Mesh gateways in non-default partitions will not have cross-datacenter
watches. They are not involved in traditional WAN federation.
