Multiple directories (up to --checkers worth) are scanned at once.
This uses much less memory than the previous scheme - only the amount
of memory needed to hold an entire directory listing of objects.
For directory based remotes the speed is unchanged.
For bucket based remotes, instead of doing one API call to list the
whole bucket, it does multiple calls, one for each pseudo directory.
However these are done in parallel so in practice this seems to speed
up directory listings.
This replaces the existing sync method as it performs faster and uses
less memory.
The old sync method is available with the temporary --old-sync-method
flag.
Fixes#517Fixes#439Fixes#236Fixes#1067
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