Do not cache _default collection in mcbp session collection cache

In order to keep consistency across SDKs it was determined that in the event of a non-existent _default collection (on the _default scope) instead of the C++ SDK raising a CollectionNotFound error, it should retry and eventually raise a timeout (the same behavior it has for user-defined collections). In order to achieve this we should remove the default setting of the mcbp session cache (code ref here).