local block adapter doesn't work with storage namespaces

[arielshaqed]

local.go Adapter.Put completely ignores the namespace of the passed-in object. So if a repo is created on some adapter, all files are actually stored in the same local directory regardless of the namespace.

Meanwhile export is unaware of this, and cannot find the file.

Apart from fixing export, we should either refuse to namespace on adapters that do not support it, or fix them to support it. (The latter may have backwards compatibility issues!)

[arielshaqed]

@guy-har fixed this in #938.