gofmt
First steps towards making inode 1 less special

Kudos to Jon Allie for the excellent suggestion!

Goal: Store inode 1 in leveldb, do a lot less special-casing of it, as
you can simply look it up.
Caveat: Some will remain, when interacting with the Drive API, but the
fuse layer, especially reads, should get a lot cleaner.

- synthesize the rootId and write it to levelDB
- special-case handling of the root's Id at inode allocation time
- remove rootId from fuse_fs.serveConn, which isn't using it
- add drive_db.DriveDB.rootId
- add locking during reinit()... seems unlikely to be necessary, but
  it's the safe thing to do.
- actually check the reverse mapping of inodeToFileId
- allocate clearFromCache more efficiently, fixing a bug along the way
- const driveFolderMimeType and copy to drive_db.go
Finish ripping out special casing of root inode

Kudos to Jon Allie for the excellent suggestion!
See previous change for goals/caveats.

- Add a bit more color to the root inode in createRoot
- Stop tracking RootInode entries in levelDB
  - rip out all the code that creates them
  - rip out all the code that queries them
- dramatically simplify getattr(), lookup() and readDir()
- drop some unnecessary special-casing and debugging from mkdir