transfer.py: Make checksum optional

Checksum takes a lot of time for large files- it should be
optimized so it can happen at the same time of the transfer, or
run in parallel, or run behind the transfer process, for each
file.

For now, we will enable it by default, but make it optional.

Bug: T156462
Change-Id: I4ec32c4fa982a6e66ec3d059bb817361fc6337d9