(ITS#9001) try_read1msg() takes O(n) steps to lookup a matching request

[ondra@openldap.org]

Full_Name: Ondrej Kuznik
Version: re24/master
OS: 
URL: https://github.com/mistotebe/openldap/tree/its9001
Submission from: (NULL) (82.10.24.68)


If a client has thousands or more requests in flight at the same time, it has to
look up the right request every time it receives a response. As the requests are
tracked in a doubly-linked list, lookup tends to take O(n) steps whichever
direction we take and things generally get out of hand from there on.

The linked branch puts them into a TAvl for quicker lookup, with another commit
to let people test as (t)avl code is part of liblutil at the moment.