Fixes: #1092

the VSL coords which are now a property of struct http.

VSL logging.  More to come.

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>