[alias-server] get count of txs that server has processed

Summary:
T2966

Depends on D13328

Get count of txs that were processed to arrive at server's current list of valid aliases. This value is required to fetch only unprocessed txs using chronik's tx history method.

Test Plan:
node index.js
Confirm processedTxCount appears in log.

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D13329