[dev] Data Structure changes

[Donald Sharp]

All -

We recently added some new data structure implementations in lib,
thanks to David Lamparter for his work here.  The new documentation is
here:

http://docs.frrouting.org/projects/dev-guide/en/latest/lists.html

And we have some commits that convert current usages to the new data structures:

7c198e4e1 lib: use DECLARE_SKIPLIST for timers instead of pqueue
c284542b6 lib: use DECLARE_LIST for thread_list
679b1649c lib: use DECLARE_HASH for qobj hash

In addition we have a PR for switching some data structures around in zebra:

https://github.com/FRRouting/frr/pull/4236

Which shows some tremendous memory savings, just by converting over.
While it may not be feasible to convert the code base quickly I would
like to ensure that the expectation is that new development uses these
new data structures.  I will be asking the question of 'why are you
not using the new data structures' in PR's that introduce new work
going forward.

thanks!

donald