by returning the name directly instead of through out param. This
function is not allowed to fail anyway.

data and metadata providers in initialize. Also, replace
get_memory_providers with get_data_memory_provider and
get_metadata_memory_provider.

Implementation of get_*_provider is supposed to be supplied
by each memory pool: this is to allow ignoring passed memory
provider (especially the metadata provider).

with METADATA and DATA provider specific errors.