Remove constraint 'context_facts cannot be added to a Context once context is referenced from a Fact'

* This was originally added to allow builing up a graph where a
  context is _fully_ defined before the context is ever
  used from a Fact.
* Removing it here, to allow more flexibility and less locking
  when entering the data (this issue becomes visible when
  actually writing the dbd_data_engine to enter data).
* If needed for populating a particular graph, this can still
  be fixed by using a "dual pass" (first collect all Contexts,
  then all Facts), or allowing enough indirection so that adding
  a ContextFact to a Context does not need to affect all individual
  Facts that are refering this Context, or other solutions ...
* this is also based on some ideas from @nathan_gs to avoid
  locking when loading up the data and  do any consistency checks
  later on if/when needed ("eventual consistency").