Drop the _owner and _lifeCycle field on internal instances

The _owner field is unnecessary since it's reachable from _currentElement.

The _lifeCycle field is unnecessary because an internal component should
not even need to exist at all if it's unmounted. It should be dereferenced
internally, and never exposed externally.

The only case where it's important is for batching updates where we
currently avoid calling performUpdateIfNecessary if it's mounted. However,
this function is already only executed "if necessary" so we just make sure
that it's not necessary after unmount by resetting all the pending fields.
Fix warning in ReactElementValidator

I got confused by the naming so I'm renaming all these `component`
callsites to refer to this object as `element`. Also, adding a unit test
to ensure that this doesn't happen again.