(ITS#6944) Add option to limit or remove operation list cache

[jorge.perez.burgos@ericsson.com]

Full_Name: Jorge Perez Burgos
Version: 2.4.21
OS: SLES 10 SP3 x86_64
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (195.235.15.243)


Our slapd uses 256 threads with conn_max_pending_auth 5000. We have seen that
with certain network traffic and conditions a huge increased in the amount of
memory. After using valgrind and a little study, we have seen slapd cache
operation structure in a list. These lists are per thread and could reach high
values, in some cases we have seen 12500 operations cached, so 256*12500 could
reach around 3GB. 

It's suggested adding an option to disable this cache or limit the growth of
this one for future versions.