Add support for pathslicing in seaweedfs backend
this actually requires quite some refactoring in the seaweedfs backend,
mostly to support the (somewhat useless) list_content()
method.
On the plus side, tests for this backend are now much more realistic,
mocking a seaweedfs Filer server instead of the WeedFiler
directly.
On the minus side, the mock object is a bit too complex for a test
helper object...
Migrated from D6492 (view on Phabricator)