New sub-module ldap.async

[Michael <mi...@st...>]

HI!

During a hopeless effort to clean up the code in web2ldap ;-) I
wrote a base class for doing e.g. stream processing and
pseudo-paging of LDAP entries with async searches.

I think it's useful for others too. Therefore I've checked in
Lib/ldap/async.py. Demo/asyncldif.py should make clear how it's
used.

Reviews welcome.

Ciao, Michael.