Make implementation of BackupableDBOptions::share_files_with_checksum strongly collision-resistant

[pdillinger]

The issue with this option is already documented:

// Only used if share_table_files is set to true. If true, will consider that
// backups can come from different databases, hence a sst is not uniquely
// identifed by its name, but by the triple (file name, crc32, file length)
// Default: false
// Note: this is an experimental option, and you'll need to set it manually
// *turn it on only if you know what you're doing*
bool share_files_with_checksum;