Merge branch 'reqid-alloc'

With these changes, charon dynamically allocates reqids for CHILD_SAs. This
allows the reuse of reqids for identical policies, and basically allows multiple
CHILD_SAs with the same selectors. As reqids do not uniquely define a CHILD_SA,
a new unique identifier for CHILD_SAs is introduced, and the kernel backends
use a proto/dst/SPI tuple to identify CHILD_SAs.

charon-tkm is not yet updated and expires are actually broken with this merge.
As some significant refactorings are required, this is fixed using a separate
merge.

References #422, #431, #463.