[PATCH 0 of 7] Refactor rrdcached connection handling and introduce URL syntax for rrd sources

This patch set refactors rrdcached connection handling. It does away
with a bunch of global variables and encapsulates all required data within
a struct. All rrdcached aware code gets changed to use the new interface.

Currently, there are some sematic changes to the current behaviour - the
last rrdcached connection does not get cached for subsequent operations -
a new connection is used for every operation.

The second big change coming with this patch set is the introduction of
URL style RRD source specifications with support for rrdcached connections.
This is done to "unclutter" the current "DEF" syntax and to provide for
better extension of the rrd source specification mechanism.

Currently missing from this patchset is the support for sql:// URLs - this is
planned for patches to come in the future.

peter stamfest