(ITS#6222) Add "unlimited" option for cachesize settings

[quanah@zimbra.com]

Full_Name: Quanah Gibson-Mount
Version: 2.4.17
OS: NA
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (75.111.29.239)


Now that we have multiple cache sizes, it would be extremely useful,
particularly with dncachesize, to be able to specify an "unlimited" setting, so
that the 2.3 behavior can be easily replicated.  Also so that people who have
systems with plenty of RAM can just set it once.

In particular, I've noticed in 2.3->2.4 upgrades, if dncachesize is particularly
small in relation to the size of the database, slapd is essentially unusable. 
I'd rather see dncachesize default to unlimited if it isn't set (2.3 behavior),
and people can tune it down from there if they need to do so.  This allows slapd
to remain usable without severely impacting the ability of slapd to function. 
It is so bad, that it becomes impossible to modify cn=config via ldap commands,
and you have to stop slapd and manually hack the cn=config text files to return
slapd to a usable state.