Rewrote netstream.c with threadpool, zlib and better queuing support

This commit represents about three weeks of work, based on research that
I've been doing somewhat idly for the past two years or so. This is the
first time that I've written a net stream API that does almost
everything I want, is both efficient and scalable, and is not a complete
and unmaintainable mess. There's still a bunch of TODO's left, but those
should be easily do-able without negatively affecting the aforementioned
properties.

A few notes:
- The net stream API hasn't been very thoroughly tested yet. There's
  pretty much guaranteed to be several bugs in there.
- globster doesn't correctly handle some cases yet, hub/connection.c
  has to be updated to reflect some changes in the API.
- There is support for zlib reading and writing, but globster doesn't
  make use of this yet.
- Although I claim that this implementation is both efficient and
  scalable, I've not done any benchmarks yet. I've not even used it to
  test file transfers. There's no doubt room for some tweaking and
  perhaps even significant performance improvements here and there.
hub/connection: Various bugfixes and simplifications
itfgen.pl: Check for change in dbo_*_set_noemit()

This makes it possible to call dbo_*_set_noemit(), _set() and
_setreply() on string properties with the property itself as argument.
hub/connection.c may do this on a reconnect timeout, and it would
previously cause a memory access violation because the property was
first free()d and then strdup()'d. With this change the property is
simply never touched at all.