GekkoFS daemon can now be restarted without losing its data

A subdirectory is no longer created by default. Therefore, a server restart uses same directory for both data and metadata. Note, that the rootdir can be cleaned with the -c argument. For multiple daemons on one machine, the new argument --rootdir-suffix can be used which creates a subdirectory within the rootdir with a user-defined name. This also allows restarting multiple daemons on one node without losing data.