connection management changes

[Howard Chu <hyc@symas.com>]

Currently we have a single select(or epoll) loop in daemon.c that lists for 
all readable and writable sockets, then passes events off to the thread pool 
for processing.

We listen for writable sockets if a write attempt returns incomplete. There's 
a pair of mutexes and condition variables used to synch up here between the 
writing threads and the listener thread. It's quite a lot of lock overhead. As 
far as I can tell the main reason we do this is so that we can stop a writer 
thread on demand instead of having it just block forever in write().

We could make the listener's job a lot easier if we only have it listen for 
readable sockets, and make each writer thread do its own poll. It would need 
to poll on two descriptors - the one it's waiting to write on, and a pipe used 
by the listener to terminate the poll. That pipe could be signalled by e.g. 
the listener writing a byte to it; all writer threads could poll for its read 
status. (One question here - if multiple threads are polling the same 
descriptor, do they all receive the wakeup event?)

--
  -- Howard Chu
  CTO, Symas Corp.           http://www.symas.com
  Director, Highland Sun     http://highlandsun.com/hyc/
  Chief Architect, OpenLDAP  http://www.openldap.org/project/