vbc's vsl_id.  When the vbc is returned, restore the original vsl_id.

Change WSLB() to VSLB() and have it take a busyobj as arg.

buffers wid with the vbc's vsl_id.  Flush the buffer when we
recycle/close the vbc.

Size controlled by param vsl_buffer which will replace shm_workspace.
The space comes out of request workspace.

locking:  Each pool fetches blocks of numbers to assign, then hands them
out one by one using the existing lock.

Session numbers are 32bit and start at zero on worker process startup.

Together with another 32bit number, local to the session, they will
be used to link VSL records into transactions.

workspace for WRW also.

difference in practice and complicates the code and increases the
size of storage data structures.

mercy of mallocs rounding algorithm.

Fix overly optimistictest-case.

conversions to uintmax_t.  Use zero to disable upper limit check.
Check separately for byte sizes larger than the architecture can do.

thread_pool_workspace is gone, adjust workspace_backend instead

Found by: Joakim Berg <flashn@glck.net>

by the operating system.

wasting partial pages with likely parameter values.

Introduce a new "backend workspace" which is used for the backend
transactions (bereq.*, beresp.*)

Together with the "client workspace" which holds the client transaction
(req.*, resp.*) this should make memory management much simpler to
understand.

The backend workspace lives in "busyobj" rather than the worker thread,
which therefore needs a lot less stack, allowing many more threads
in the same space.

Experiments with thread_pool_stack are encouraged, I wouldn't be
surprised if 32k is now enough.

The per-thread workspace is still around, known as the "auxiallary
workspace", but it is down to 256 bytes and it is only used to hand
the acceptor information off to worker threads.

Once the dust settles, I may remove it entirely, but I can see a
value of having a small amount of fast memory in a thread, so I may
also keep it.  No promises.

Ohh, and I removed a couple of small memory leaks which were only
present in -trunk, as a result of a git-mis-merge.

trust that to be the same throughout the request.

update the desired size of objects from the guard thread.

Rename the gzip_stack_buffer param to gzip_buffer.
Remove the gzip_tmp_space param.

loose the dedicated "pending" buffer and just use the front of the
input buffer.  This simplifies the code quite a bit.