Verify only 1mb of existing LFS object on download

Merged Igor Drozdov requested to merge id-link-existing-lfs into master

What does this MR do?

Related issue: #214211 (closed)

The merge request is based on this idea: #214211 (comment 345221514)

If LfsObject already exists, we check whether the first 1mb of the existing file is the same as 1mb of the remote file.

We assume that first 1mb is not predictable and rely on this in terms of security concerns: #214211 (comment 348037156)

Edited by Igor Drozdov