Revise BsonFilter docs/signature and corresponding test Filter
CriteriaMerger should combine criteria as $and clauses

Filter, mapping, reference, and userland query criteria is now combined as $and clauses. Conflicting criteria will not overwrite anything, so you are more likely to get no results rather than inadvertently circumvent a filter or mapping criteria.

CriteriaMerger::merge() is no longer static, so the CriteriaMerger object can be injected into FilterCollection or DocumentPersister (defaults to a new object of the base class).

GH596Test needed to be updated, among others, to expect the new prepared format.
Filter criteria should always be merged after discriminator criteria

Since merging of filter criteria may result in the original query being wrapped in "$and", the logic in addDiscriminatorToPreparedQuery() can not easily check whether the discriminator field is present in the original query. This can be mitigated by applying discriminator criteria before filter criteria.
Fix preparation of logical clauses and operators w/o typed args

DocumentPersister::prepareQueryOrNewObj() should not recurse on the array of clauses for logical operators, but rather the individual clauses.

Additionally, prepareQueryElement() should not perform identifier conversions for query operators whose arguments need no conversion (e.g. $exists). Other operators, such as $in, should still have their arguments prepared as identifier values.