Summary:
Currently IngestExternalFile() fails when its input files' ranges overlap. This condition doesn't need to hold for files that are to be ingested in L0, though.
This commit allows overlapping files and forces their target level to L0.
Additionally, ingest job's completion is logged to EventLogger, analogous to flush and compaction jobs.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5539
Differential Revision: D17370660
Pulled By: riversand963
fbshipit-source-id: 749a3899b17d1be267a5afd5b0a99d96b38ab2f3