[Cashtab] [Chronik] Airdrop block height retrieval

Summary: Swap out existing bch-api calls retrieving the latest block height with the chronik client.

Test Plan:
npm start
note the debug statement in Airdrop.calculateXecAirdrop() to display the latest block height upon airdrop execution and ensure it matches the latest block shown on explorer.e.cash
ensure no regression errors with airdrop calculation

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12181