[Proposal] Remove DAV properties cache in ra_serf

Hi,

Currently ra_serf caches *all* DAV properties retrieved using PROPFIND
in session pool. This was attempt to reduce number of PROPFIND
requests. But current implementation has several problems:
1. Unlimited memory usage: current implementation stores all
properties, so svn ls -R command can easily eat 150 mb of memory.
2. Very low cache hit due the fact that cache doesn't support caching
"allprop" requests. For some operations there is no hits at all.
3. Current implementation caches properties that may change between
requests, like URL to youngest revision.

Of course these problems can be fixed, but I'm not sure that we need
this code. Since in svn 1.7 we have HTTPv2 which doesn't use so many
PROPFIND requests that we tried to reduce using DAV properties cache.

So I'm propose just to remove DAV properties cache from ra_serf.
Objections? Comments?

PS: See attached patch in case you'd like test performance.

-- 
Ivan Zhakov