Can't safely use uint32 for locker since we need to decrement the count,
which requires loading the unit and doing some math, which is inherintly
racey.
Instead use Int32 which we can safely use with atomic and AddInt32 with
`-1`
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
Uses finer grained locking so that each volume name gets its own lock
rather than only being protected by the global lock, which itself needs
to be unlocked during cetain operations (`create` especially`)
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
