This also changes the wsi_children array to be indexed by subchannel - 2,
non-existent channels are NULL in there and highest_child_subchannel
is a highwater mark for the highest subchannel ever used. That way we
can immediately get the wsi for a subchannel at the cost of some extra
sparse skipping during management.
This also takes care of scanning for empty slots on allocation and
NULLing on close of subchannel instead of deletion.
Signed-off-by: Andy Green <andy@warmcat.com>