gluon-mesh-babel: Reduce load in respondd

[christf]

This reduces the load by:

• querying the default route using netlink instead of parsing it from babel
• instead of dump, use monitor and a separate thread to obtain the babel neighbours

• [ ] resolve TODO in a different PR this on is meant to fix a problem: include traffic stats for all mesh interfaces

[rotanid]

i just noticed that this PR contains parts that are not directly related to "reduce load", maybe you can split this apart and get parts merged faster and also reduce this PR's size?

[christf]

@t-8ch thank you for your review. Were all of your comments considered as intended?

[christf]

unfortunately libjson-c 0.13 is not yet in relevant gluon versions for a while. only 0.13 includes json_object_deep_copy(). Because of this I introduced a fallback that should work with 0.12. Testing is in progress.

[rotanid]

this needs a rebase now @christf ... and somehow it's still missing a review with C+Gluon skills after that :(

[christf]

rebase is done.

[neocturne]

See https://github.com/christf/libbabelhelper/pull/9 for current progress on libbabelhelper cleanup.

[rotanid]

@christf needs rebase?

[blocktrron]

Closed because of #3105