Rename emap interface method names

[tsachiherman]

What ?

As part of having the shared validitywindow implementation being used within the dsmr, the entries that would be contained would be better off implementing the same interface.

The dsmr's interface ( i.e. Tx ) already using GetID() and GetExpiry(), which seems to be "better" compared to the emap.Item's ID() and Expiry() as there would clash with member names.

This PR is a followup on #1782, and part of Generalize expiry based de-duplication

[joshua-kim]

One pro of this is that the interface function name is unlikely to collide w/ field names (e.g ID is a common field name).

[aaronbuchwald]

We should be consistent throughout the codebase between using ID() and GetID() imo. We are bound to use ID() to implement snowman.Block interface from AvalancheGo, but could push everything else to switch to GetID(), which would be fine imo.