client: Snapshot cephfs APIs

client: Snapshot cephfs APIs

Introduce calls to create and delete a snapshot. The difference
b/w this and the traditional mkdir() call to create a snapshot
is the additional metadata field that can be persistent along
side a snapshot (on creation). However, rmsnap() does nothing
special and is included for completeness.

Additionally, get_snap_info() API allows to fetch snapshot info
which includes things like snaphot ID and metadata (if any, which
was set via mksnap() API).

Signed-off-by: Venky Shankar <vshankar@redhat.com>