[chronik-plugin] Rename token to etoken

Summary: The token.py python module conflicts with the built-in python module of the same name. This is confusing and also causes flake8 to report an error, preventing the linters from doing their magic.

Test Plan:
With Chronik and the plugins built:

ninja check-functional

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D16830