Add db specific node class registry

[mariusconjeaud]

Today, neomodel uses a single node class registry, to prevent collision of labels inside the database.

So this would not work :

class PatientOne(AsyncStructuredNode):
        __label__ = "Patient"
        name = StringProperty()

class PatientTwo(AsyncStructuredNode):
        __label__ = "Patient"
        identifier = StringProperty()

This PR adds a database-specific node class registry so that in the context of multiple databases, two different StructuredNode classes can map into the same label, but in different databases :

class PatientOne(AsyncStructuredNode):
        __label__ = "Patient"
        __target_databases__ = ["db_one"]
        name = StringProperty()

class PatientTwo(AsyncStructuredNode):
        __label__ = "Patient"
        __target_databases__ = ["db_two"]
        identifier = StringProperty()

[codecov]

Codecov Report

Attention: Patch coverage is 96.55172% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 91.47%. Comparing base (7c6662a) to head (53ff6fc).

Files	Patch %	Lines
neomodel/async_/core.py	96.00%	1 Missing ⚠️
neomodel/sync_/core.py	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           rc/5.3.2     #806   +/-   ##
=========================================
  Coverage     91.47%   91.47%           
=========================================
  Files            34       34           
  Lines          4574     4600   +26     
=========================================
+ Hits           4184     4208   +24     
- Misses          390      392    +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sonarcloud]

Quality Gate failed

Failed conditions
18.8% Duplication on New Code (required ≤ 3%)

See analysis details on SonarCloud