implement maybe_wrap_in_index for EA Mixin dispatch
Implements `maybe_wrap_in_index`, shows the easiest places where it can be used.  If merged, the follow-on will be to move the affected methods to the EA mixin classes, leaving just the dispatch/wrapping behind.

Implements `_local_timestamps` on the `DatetimeArrayMixin`, since that will be needed before we can move the comparison operators among others.