Log more information in run_check_repl_dbhash_background.js

run_check_repl_dbhash_background.js has three steps:

1. Use a failpoint to stop discarding history of updates in WT across all nodes.
2. Negotiate a timestamp that's in the history of all nodes.
3. Perform and compare results of a dbhash check on all nodes at the negotiated timestamp.

More logging should be added to steps one and two.