return path from requested prefix instead of root when fetching namespace

[pkasprzyk]

ref. https://neptune-labs.atlassian.net/browse/NPT-10417

before:

import neptune.new as neptunerun = neptune.init()
run['model/params'] = {'a': 3}
run.wait()
params run['model/params'].fetch() == {'model': {'params': {'a': 3}}}

after:

import neptune.new as neptunerun = neptune.init()
run['model/params'] = {'a': 3}
run.wait()
params run['model/params'].fetch() == {'a': 3}

Also refactored fetch_atom_attribute_values to make it more unit-testable.