Get rid of the complex logic of choosing the largest saved version from
previous epoch for the oldest epoch. Instead, use the begin version now
available from log system.
Sometimes the backup worker has not updated progress to the system space and a
master recovery happens. As a result, next epoch doesn't know the progress of
previous ones. This change is to check for such missing gaps and fill them with
the whole range [startVersion, endVersion).
The code is refactored into BackupProgress.actor.* to consolidate backup
progress processing for the master server.
Jingyu Zhou