R1:6dc341be8708

As Harbormaster logs are processed, build a sparse map of byte offsets to line numbers

Summary:
Depends on D19138. Ref T13088. When we want to read the last part of a logfile //and show accurate line numbers//, we need to be able to get from byte offsets to line numbers somehow.

Our fundamental unit must remain byte offsets, because a test can emit an arbitrarily long line, and we should accommodate it cleanly if a test emits 2GB of the letter "A".

To support going from byte offsets to line numbers…