First half of Issue #74.
BIB & session are now red-black trees, and passed the unit tests.
Integrated into Jool however, they're still not expected to work, at least
because the remove functions now probably expect the nodes to already belong to
the trees.

Also, there's a wrapper for the icmp send functions, because it needs to avoid
the send with locally generated packets.
Merge branch 'fragmentation' of github.com:NICMx/NAT64 into fragmentation