New plugin rrdtool_createonly

Hello,

This is a new attempt to create a fix for issue #75 if you are using rrdtool/rrdcached.

This replace my hack of rrdtool plugin (#145).

Features : 
- create rrd files
- cache : instead of checking the presence of a rrdfile with stat() every time, it stores its presence in a cache for a while (for example, 1 hour). -> less I/O.
- this is no more a hack of an existing plugin that was not designed for such a job.

Regards,
Yves