Summary:
Delete triggered compaction (DTC) for universal compaction style with ```num_levels = 1``` has been disabled for sometime due to a data correctness bug. This PR re-enables it with a bug fix. A file marked for compaction can be picked, along with all L0 files after it as the compaction input. We stop adding files to the input once we encounter a file already being compacted (the original bug failed to check the compaction status of the files).
Tests:
Add unit tests to ```compaction_picker_test.cc```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7224
Reviewed By: ajkr
Differential Revision: D23031845
Pulled By: anand1976
fbshipit-source-id: 9de3cab5f9774cede666c2c48d309a7d9b88a505
f308da5273