Feat/database as storage (#23)

* add _status to trial

* save status in _status when queried

* add new database implementation plus its tests

* adapt asha + test

* improve init

* update constants

* update logger with the new database implementation

* add trail_id argument to read_trials

* make Logger a singleton, get rid of env variables

* only import SlurmCfg

* new backend implementation

* remove storage

* adapt experiment

* adapt readme test

* fix slurm test

* small fix

* update type hints + docstrings

* update readme