Improve tests of the seaweedfs backend
tests for this backend are now much more realistic,
mocking a seaweedfs Filer server instead of the WeedFiler
directly.
And actually fix a bug in WeedFiler.get()...
Also remove tests for each compression algo, keeping only one is enought.
Also improve a bit the seaweedfs backend
Migrated from D6517 (view on Phabricator)