Support readonly (filesystem) mode

Add support for the server to start without creating or writing any files. If the filesystem is mounted, or files are, read-only this mode would allow starting the server.

This mode would also be useful when using tools in dbpath mode where they read the raw database files, instead of working in client/server mode.

For people doing snapshot backups (like LVM) which can be mounted read-only this option would allow running the server against the snapshot without making an additional copy. This is a little more tricky if there are journal entries to commit before the files are useable or if there is a lock file and no journal (see SERVER-7861).