Storage: use names instead of uuids in interface

Instead of exposing and requiring engines and users to use uuids,
storage now provides more convenient interface that allows to do
anything with task by its name.

Storage is now the entity that maintains name-to-uuid mapping. To
emphasize that, find_by_name method (now unused) was removed from
FlowDetail.

Breaking change: this will break clients that use storage to manipulate
tasks. Clients that use engine.storage only to fetch results with
'fetch' and 'fetch_all' methods (which is the recommended way) are not
affected.

References blueprint task-executor
Co-authored-by: Ivan A. Melnikov <imelnikov@griddynamics.com>
Change-Id: Icbdb745c1ed84c5ac21c6f3a275f32c0b2939d90