Summary:
Implemented a subcommand of sst_dump called identify, which determines whether a file is an SST file or identifies and lists all the SST files in a directory;
This update also fixes the problem that sst_dump exits with a success state even if target file/directory does not exist/is not an SST file/is empty/is corrupted.
One test is added to sst_dump_test.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6943
Test Plan: Passed make check and a few manual tests
Reviewed By: pdillinger
Differential Revision: D21928985
Pulled By: gg814
fbshipit-source-id: 9a8b48e0cf1a0e96b13f42b690aba8ad981afad3
119b26fac0