remote transfer slow for unversioned data

[dberenbaum]

    w/threadpoolexecutor and the `cats-dogs` dataset:

default remote:

time dvc push -r s3-unversioned
2801 files pushed
dvc push -r s3-unversioned  41.37s user 7.50s system 10% cpu 7:56.26 total

time dvc pull -r s3-unversioned
A       cats-dogs/
1 file added and 2800 files fetched
dvc pull -r s3-unversioned  12.03s user 4.40s system 21% cpu 1:14.68 total

version_aware = true remote:

time dvc push -r s3-versioned
2800 files pushed
dvc push -r s3-versioned  21.65s user 3.40s system 12% cpu 3:13.01 total

time dvc pull -r s3-versioned
A       cats-dogs/
1 file added and 2800 files fetched
dvc pull -r s3-versioned  11.19s user 4.03s system 20% cpu 1:15.42 total

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Not sure why versioned remote push performs so much faster than unversioned on my machine after these changes, it may be due to the same listing performance problems noted in the gc issue iterative/dvc#5961 (comment). (we don't do full remote listing for versioned remotes)

Originally posted by @pmrowla in #246 (comment)