libsnapshot: require ex lock for {Create}{CowImage,Snapshot} / DeleteSnapshot

Create / Deleting the COW image / snapshot changes states, so it makes
sense to require an exclusive lock before doing so. If caller doesn't hold
an exclusive lock, parallel calls to MapCowImage / MapSnapshot / UnmapCowImage /
UnmapSnapshot may have weird results.

Test: libsnapshot_test

Change-Id: I4be660df1059ec24144f8baf43a1c8c05d9e372b