remote: use progress bar for remote cache query status during dvc gc

[pmrowla]

As noted in #3532

both local and ssh have their own overridden cache_exists() implementation and neither one uses list_cache_paths() at all during cache_exists(). The only time list_cache_paths() would get called for either local or ssh remotes would be for RemoteBASE.all() during a dvc gc call.

all() does not currently display any progressbars at all, but now that I'm thinking about this, it probably should

RemoteBASE.all() can use the same remote size estimation behavior as cache_exists() so that dvc gc displays actual progress/status when iterating over the full list of remote checksums. This will make gc behavior consistent with push/pull