When shutting down a process, free resources early

Due to lingering connections, shutting down a process may take a very
long time. Free all recycled pools early in the hope that we can already
give some memory back to the OS.


rename some variables to be more descriptive

pid -> pslot
tid -> tslot
remove unused 'sd'


Terminate keep-alive connections when dying

When shutting down a process gracefully, terminate keep-alive connections so
that we don't get any new requests which may keep the dying process alive
longer.



Exit threads early during shutdown

During graceful shutdown, if there are more running worker threads than open
connections, terminate some threads. This frees resources faster, which may be
needed for new processes.



Exit threads early during shutdown, part 2

Follow up to r1738466: During graceful shutdown, when the listener thread is
closing a connection, it needs to wake up a worker thread so that it may
terminate.


Submitted by: sf
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1772334 13f79535-47bb-0310-9956-ffa450edef68