refactor ABI for arbitrary attribute/entry/callback registration; propagate pointers to Operation and SlapReply structures to entry create/update/modify callbacks; cleanup

Fix suggested by Peter Marshall <peter@adpm.de>

work around the last issue: some times the result times out while the other party is not willing to send any response; give it a second chance

the connection is busy. Probably need to set a queue limit here and
abandon the psearch if the limit is reached.

attempting to write the response on the network. (This should probably
be the default behavior anyway, since network hangs of this sort can
cause problems for any backend that has entry locks.)