ITS#6152 pcache enhancements

[Howard Chu <hyc@symas.com>]

Just starting to look at implementing Time-To-Refresh, and wondering how it 
impacts other parameters.

proxysavequeries allows queries to be saved and reloaded across restarts. 
Currently it will only reload non-expired queries. The implication of setting 
the offline mode is that expired entries may remain in the cache. Should they 
in fact be reloadable too?

The proxytemplate directive has gotten a bit cumbersome, with optional 
negative TTL and sizelimit TTL added on. This would be the natural place to 
add the TTR config as well. Should there be independent TTR for negative and 
sizelimit too? (And it seems we should move from positional arguments to named 
arguments here...)

Given the existence of a TTR feature, I'm less concerned about the 
proxysavequeries behavior. One should simply set a long TTL and set TTR as 
needed to keep the cache fresh.

As a side point, I'd prefer more consistency in the config keyword names, we 
should have used "pcache" as the prefix for all the keywords...

pcache <database> ...
pcacheattrset <index> ...
pcachemaxqueries <queries>
pcachevalidate <bool>
pcachepersist <bool>
pcacheset template=<string> attrset=<index> ttl=<secs> [negttl=<secs>] ...
pcacheposition (head|tail)

--
  -- Howard Chu
  CTO, Symas Corp.           http://www.symas.com
  Director, Highland Sun     http://highlandsun.com/hyc/
  Chief Architect, OpenLDAP  http://www.openldap.org/project/