fleetctl stop is slow with large number of units

[iamruinous]

Similar to #1340, stop and unload are also incredibly slow when a large number of unit files exist (~2500).

Submitting a pull request in a bit.

[wuqixuan]

I totally agree that no need get all unit information.

[ivankennethwang]

+1

[dongsupark]

As described in #1344 (comment), using Unit() instead of Units() will make the performance rather worse. So I'll close this issue for now.
If anyone has a better idea to improve the performance in a generic manner, please create another PR to do so. Thanks.