locks: provide a file lease method enabling cluster-coherent leases

Currently leases are only kept locally, so there's no way for a distributed
filesystem to enforce them against multiple clients.  We're particularly
interested in the case of nfsd exporting a cluster filesystem, in which
case nfsd needs cluster-coherent leases in order to implement delegations
correctly.

Also add some documentation.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>