Fix uninitialized fields in file metadata

[ajkr]

This is a quick fix for the uninitialized bugs in LiveFileMetaData and SstFileMetaData that were uncovered in #4686.

Test Plan:

• make check -j64
• manual testing for read counts since the behavior is non-deterministic

In [23]: lookup_key = next(filter(lambda file: file['name'] == '/000973.sst', db.get_live_files_metadata()))['smallestkey']

In [24]: for i in range(100000):
    ...:     db.get(lookup_key)
    ...:

In [25]: next(filter(lambda file: file['name'] == '/000973.sst', db.get_live_files_metadata()))['num_reads_sampled']
Out[25]: 109568

[ajkr]

#4686 attempts to create a long-term fix and will take a bit longer to figure out. I'd prefer to land this hopefully uncontroversial PR in the meantime.

[abhimadan]

LGTM, thanks for catching this

[siying]

Thank you for fixing it. Too bad I didn't realize this obvious bug when I introduced the counter.

[riversand963]

LGTM. I hope we can address this problem more thoroughly in your separate PR #4686

[facebook-github-bot]

@ajkr is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.