locking.LockSet: Replace boolean parameter with constants

Upcoming changes will add opportunistic locking to “locking.LockSet”.
Doing so will require additional code in “LockSet.__acquire_inner”, at
which point the existing “want_all” parameter does not always apply.
This patch replaces it with a “mode” parameter. For opportunistic
locking a new mode will be added.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>