expose a less copy-heavy interface from Reflector

[clux]

Currently all Reflector::read calls do a full copy:

https://github.com/clux/kube-rs/blob/3b14161baefe64ba308344e886b186aa4fbb0c7f/src/api/reflector.rs#L133-L149

this is probably not what all users want (see #57).

Alternatives so far:

• wrap self.cache in an Arc and hand out clones of that
• make read be the equivalent of into_iter and expose an alternative iter equivalent

The first sounds tricky to get right depending on how people drive the reflector (borrow-checking might get harder to satisfy) when one thread polls and other threads want to read. Attempts at making this better is welcome.

[clux]

Well, this was done in the linked commit really. Closing for now until there's an actual issue with that interface.