Summary:
This was requested by a customer who wants to proactively monitor whether any valid backups are available. The existing performance was poor because Open() serially reads every small meta-file (one per backup), which was slow on HDFS.

Now we only read the minimum number of meta-files to find `max_valid_backups_to_open` valid backups. The customer mentioned above can just set it to one.
Closes https://github.com/facebook/rocksdb/pull/2151

Differential Revision: D4882564

Pulled By: ajkr

fbshipit-source-id: cb0edf9e8ac693e4d5f24902e725a011ed8c0c2f