Problem with block processing after restarting the backend

Problem: balance tracker errors out if it is restarted.

Steps to reproduce:

• Start HAF with 5M blocks
• Set up the balance tracker's database
• Run btracker_app.main('btracker_app', 0);
• Wait for it to finish processing all the blocks
• Stop the process and run btracker_app.main('btracker_app', 0); again.

Expected result: balance tracker waits for new blocks

Actual result: balance tracker tries to reprocess the blocks and fails while processing block range <905601, 905700>

Full logs: processing.2.log.7z