More will likely be dropped in the future, but drop the obvious
ones we aren't using, for now; especially since some of them are
set at ->new time and unavoidable.
This saves 579 bytes per-client on my 64-bit Debian stable
system as measured by Devel::Size::total_size from
PublicInbox::HTTP::event_read. This adds up in C10K or C100K
situations.
Things we drop are:
* corked - MSG_MORE requires fewer syscalls
* read_push_back - tried to use it, ate CPU with slow clients
* IP/port fields - accept() already returns what we care about