oplog default size must be differently calculated for inMemory storage engine

If oplog size is not specified, we calculate it using default based on free space on disk. This makes little sense for inMemory storage engine and will most likely never be the right size.

We should either fail to start if oplog size is not specified explicitly for a replica set member on startup, or we should calculate a different default, based on memory size (like we do cache) not based on disk size (and maybe give a warning in the logs that the size was not specified).