Fix a bug in bf_search regarding boolean values

[MaxDall]

There was a nasty bug in LD.bf_search basically rendering the parsing of boolean values unusable. This PR fixes this issue by introducing a new sentinel value and fixing some logic bugs.

One problem this could cause is that now the free_access attribute finally works as intended and this may cause some trouble since the publishers won't set it honestly.

Update: I opened another PR #423 to address this

[addie9800]

Is there a reason to create the _sentinel object instead of using None?

[MaxDall]

Is there a reason to create the _sentinel object instead of using None?

The sentinel is unique whereas None isn't. One could not differentiate if the value of a key was None or not present at all and thus default would be returned for all None values, besides it only should be returned when the key isn't present.

[addie9800]

Thanks for the explanation, that makes sense. Looks good and thanks for catching this 👍