With this change it'll be possible to register other lock information
providers. One usecase for this are job dependencies, which can be shown
in the output of “gnt-debug locks”, too.

The lock monitor is changed to accept more than one return value from
the function providing the information. Unfortunately it's hard to keep
weak references to bound methods, so that I settled on keeping a weak
reference on the object instead (see note in docstring).

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