Support exists checking on download by checksum

[bmflynn]

Currently, files already existing on disk by name are skipped when downloading. This does not work with files that get replaced in CMR but have the same name, such as NRT products that do not have a creation time in the filename.

We could add an opt-in flag that will force the exists check to compare the remote file metadata checksum against a local file in the case that the checksum is available, the algorithm is supported, and the file exists locally.