Implement lock names for debugging purposes

This patch adds lock names to SharedLocks and LockSets, that can be used
later for displaying the actual locks being held/used in places where we
only have the lock, and not the entire context of the locking operation.

Since I realized that the production code doesn't call LockSet with the
proper members= syntax, but directly as positional parameters, I've
converted this (and the arguments to GlobalLockManager) into positional
arguments.

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