src/utils_rrdcreate.[ch]: Implement the srrd_create_async() function.

src/utils_rrdcreate.[ch]: Implement the srrd_create_async() function.

This new functionality, which is also exported by the rrdtool and
rrdcached plguins, allows to create new RRD files in the background.
This avoid blocking write threads on the RRD creation, when hundreds of
files need to be created.

See also Github issues #243 and #244.