platsupport: remove redundant helper function

- remove tqueue_next as it was only used once and actually makes the
code more complex.

platsupport: fix invalid comparator in tqueue

- Subtraction is not compare for unsigned values :(
- also remove id field from comparator, not required as cmp
does not reflect node equality in sglib (it's not java)

platsupport: add time manager + local implementation

This is an interface for time services, plus a default implementation of a local time
manager.

The time manager takes an ltimer and uses it to provide the
current time and timeouts which are called as callbacks. It provides a
mapping between ids (generated by the time manager) and callbacks.

Simply put this time manager interface uses an ltimer and the timeout data
structure to provide local timeout multiplexing.