Disabling file system storage no longer works

Previous versions of cbgt had the ability to disable file system storage by not setting the manager's dataDir (MB-36085).  In addition to the changes on this ticket, this also relied on bypassing the initial load from the data dir (on manager.Start), by setting the 'managerLoadDataDir' option to something other than the recognized options (sync/async/empty).  Sync Gateway was setting this to 'false'. 

The removal of the sync/async option (https://github.com/couchbase/cbgt/commit/b3a5fe28b0b4d0cb8e4442b98d1a2f2f8f5a80c5) has broken this on the latest cbgt version, which we want to move to in order to pick up the latest gocbcore DCP feed support.