The change streams have been implemented assuming that the BSONObj ownership is kept by the MatchExpression.

To reduce the overheads of copying BSONObj into MatchExpressions, we aim to remove the _backingBSON from the MatchExpression. The MatchExpression will compare against a reference of an already existing BSONObj which survives the query. 

This ticket will update the design of Stream Helpers:

i. Match expressions depend on already existing BSONObj.

ii. Ensure all BSONObj used in MatchExpressions are in scope.

iii. Simplify where possible the building of MatchExpressions.