Add limit parameter to $or operator to provide control of the number of queries that will be executed in parallel

Decide whether or not this is necessary and if it is then implement.

The current $or operator behaviour executes all clauses of an $or in parallel. The default behaviour should remain unchanged, however is it possible to add a control to limit the number of parallel queries (individual $or clauses executed) launched. With a poorly designed query this can cause performance issues at the mongod level and at a sharded level can trigger timeouts from the mongos processes.