[token-server] Add endpoint to check address eligiblity for token rewards

Summary: Add a server endpoint that checks if given address is eligible for token rewards. If the address is not eligible, return a timestamp at which it will become eligible.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack, PiRK

Reviewed By: #bitcoin_abc, emack, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15678