event, worker: runtime pool.

MPMs event and worker both need a dedicated pool to handle the creation of
the threads (listener, workers) and synchronization objects (queues, pollset,
mutexes...) in the start_threads() thread, with at least the lifetime of
the connections they handle, and thus survive pchild destruction (notably
in ONE_PROCCESS mode, but SIG_UNGRACEFUL is concerned too).

For instance, without this fix, the below backtrace can happen in ONE_PROCCESS
mode and a signal/^C is received (with active connections):

Thread 1 "httpd" received signal SIGSEGV, Segmentation fault.
(gdb) bt
#0  <BOOM>
#1  0x00007ffff7c7e016 in apr_file_write (thefile=0x0, ...)
                                          ^ NULL (cleared)
                       at file_io/unix/readwrite.c:230
#2  0x00007ffff7c7e4a7 in apr_file_putc (ch=1 '\001', thefile=0x0)
                                                      ^ NULL (cleared)
                       at file_io/unix/readwrite.c:377
#3  0x00007ffff7c8da4a in apr_pollset_wakeup (pollset=0x55555568b870)
                                              ^ already destroyed by pchild
                       at poll/unix/pollset.c:224
#4  0x00007ffff7fc16c7 in decrement_connection_count (cs_=0x7fff08000ea0)
                       at event.c:811
#5  0x00007ffff7c83e15 in run_cleanups (cref=0x7fffe4002b78)
                       at memory/unix/apr_pools.c:2672
#6  0x00007ffff7c82c2f in apr_pool_destroy (pool=0x7fffe4002b58)
                                            ^ master_conn
                       at memory/unix/apr_pools.c:1007
#7  0x00007ffff7c82c12 in apr_pool_destroy (pool=0x7fff08000c28)
                                            ^ ptrans
                       at memory/unix/apr_pools.c:1004
#8  0x00007ffff7c82c12 in apr_pool_destroy (pool=0x555555638698)
                                            ^ pconf
                       at memory/unix/apr_pools.c:1004
#9  0x00007ffff7c82c12 in apr_pool_destroy (pool=0x555555636688)
                                            ^ pglobal
                       at memory/unix/apr_pools.c:1004
#10 0x00005555555f4709 in ap_terminate ()
                       at unixd.c:522
#11 0x00007ffff6dbc8f1 in __run_exit_handlers (...)
                       at exit.c:108
#12 0x00007ffff6dbc9ea in __GI_exit (status=<optimized out>)
                       at exit.c:139
#13 0x00007ffff7fc1616 in clean_child_exit (code=0)
                       at event.c:774
                                  ^ pchild already destroyed here
#14 0x00007ffff7fc5ae4 in child_main (child_num_arg=0, child_bucket=0)
                       at event.c:2869
...


While at it, add comments about the lifetimes of MPMs pools and their objects,
and give each pool a tag (e.g. "pchild" accordingly to other MPMs).



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1835845 13f79535-47bb-0310-9956-ffa450edef68