Multiple DaskStorage instances

[jrbourbeau]

This allows multiple DaskStorage instances with the same type of underlying Optuna storage class by adding a new unique name identifier. Today if one does:

import dask_optuna

s1 = dask_optuna.DaskStorage()
s2 = dask_optuna.DaskStorage()

then the OptunaSchedulerExtension registered on the scheduler will store only a single Optuna InMemoryStorage class in it's .storages dictionary.