cmd/upspin: do not read whole files into memory in tar
cmd/upspin: do not read whole files into memory in tar

Use Client's Open and Create methods instead of Get and Put.
This effectively limits the peak memory usage of a tar/untar operation
to upspin.BlockSize plus epsilon, while before this change you required
as much memory as the largest file you were tarring/untarring.

Change-Id: Ib7c5a2011e73e3d6089b0df9ad353be231437052
Reviewed-on: https://upspin-review.googlesource.com/17460
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin