lighttpd/DAV support

[Brad Fitzpatrick]

Mogilefsd now supports using lighttpd (or any other DAV server presumably)
as your mogstored storage nodes.

Docs are checked into subversion:

    http://code.sixapart.com/svn/mogilefs/trunk/server/doc/lighttpd.conf

Additionally, performance enhancements were made in mogilefsd to support
this use case:

 -- caching that the special 'side channel' socket to mogstored isn't
    available.  it's primarily used for doing size requests to storage
    node files, but it falls back to a HEAD request over HTTP if not
    available.

 -- caching the socket that HEAD requests go on, doing Keep-Alive.

So now the HEAD requests just about as fast as the side channel stuff.

The only problem with lighttpd at this point is nothing is writing out the
per-device usage files, so mogilefsd can't weight new files based on
device capacity/usage.   That'll be fixed soon enough, with either a
separate tool or a special mode to mogstored to write these out without
running in webserver mode (and not requiring Perlbal/Danga::Socket
libraries to be installed)

- Brad