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