Summary:
In case of huge db backup infromation about progress of downloading would help.
New callback parameter in CreateNewBackup() function will trigger whenever a some amount of data downloaded.
Task: 8057631
Test Plan:
ProgressCallbackDuringBackup test that cover new functionality added to BackupableDBTest tests.
other test succeed as well.
Reviewers: Guenena, benj, igor
Reviewed By: igor
Subscribers: dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D46575