Add MongoDB adapter for Zend\Cache
Cleaned up comments, try/catch blocks and if conditions
Skip MongoDBTest if mongo not enabled, check mongo return value === (double) 1
Additional MongoDB adapter improvements
 * suggest mongofill/mongofill
 * Instantiate MongoCollection in MongoDBResourceManager
 * Allow for all Mongo/MongoClient options to be passed to constructor
 * Implement flushable interface
 * Index collection on 'key' when created
Cleanup use statements/remove array short syntax
Allow Mongo polyfill if Mongo extension is not loaded
Remember to pass driverOptions to Mongo|MongoClient is set
Better handling of namespacePrefix & expires field, additional checks on returned cached items, add MongoID as supported metadata
#6078 - reduce visibility on internal API methods, fix minor docblock issues
#6078 - simplifying implementation details, removing redundant checks
#6078 - minor CS fix (alignment)
#6078 - adding missing dynamically defined property to `CommonAdapterTest`
#6078 - minor CS fixes/cleanups in `MongoDBOptions` implementation
#6078 - avoiding by-ref assignments where not needed, cleaning up implementation details, simplifying imports and docblocks
Merge branch 'feature/#6078-mongodb-cache-storage' into develop

Close #6078