Make sure if a DB Session got created for a request, it stays unique per request

[6543]

to prevent deadlocks, etc...:

• use context for all function calling package model and model itself
  • more context for models  #19511
  • ...
• add func that can extract an open session from context if there is one or open a new one
• be able to make sure, if db.TxContext() was already opened, a function within the stack does not open a second one again