make trial import lazy to avoid cyclic dependencies

[Delaunay]

Storage factory implicitly import backends when importing orion/storage/base which cause a cyclic dependency

Solution is to break the cycle by importing Trial lazily in the backends

Traceback (most recent call last):
  File "/home/setepenre/work/olympus/olympus/report/dashboard.py", line 13, in <module>
    from orion.storage.base import Storage
  File "/home/setepenre/work/orion/src/orion/storage/base.py", line 213, in <module>
    class Storage(BaseStorageProtocol, metaclass=SingletonFactory):
  File "/home/setepenre/work/orion/src/orion/core/utils/__init__.py", line 57, in __init__
    super(SingletonType, cls).__init__(name, bases, dictionary)
  File "/home/setepenre/work/orion/src/orion/core/utils/__init__.py", line 112, in __init__
    entry_point.load()
  File "/opt/anaconda3/lib/python3.7/site-packages/pkg_resources/__init__.py", line 2443, in load
    return self.resolve()
  File "/opt/anaconda3/lib/python3.7/site-packages/pkg_resources/__init__.py", line 2449, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/home/setepenre/work/orion/src/orion/storage/legacy.py", line 18, in <module>
    from orion.core.worker.trial import Trial
  File "/home/setepenre/work/orion/src/orion/core/worker/__init__.py", line 17, in <module>
    from orion.core.worker.consumer import Consumer
  File "/home/setepenre/work/orion/src/orion/core/worker/consumer.py", line 20, in <module>
    from orion.core.worker.trial_pacemaker import TrialPacemaker
  File "/home/setepenre/work/orion/src/orion/core/worker/trial_pacemaker.py", line 12, in <module>
    from orion.storage.base import get_storage
ImportError: cannot import name 'get_storage' from 'orion.storage.base' (/home/setepenre/work/orion/src/orion/storage/base.py