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.
rrdtool, rrdcached plugin: Whitespace fix.
src/utils_rrdcreate.c: Free allocated memory in all error handling cases.
src/utils_rrdcreate.c: Add folding markers.
src/utils_rrdcreate.c: Lock files to be created.

This works around an issue where RRDtool crashes due to two threads
trying to create the same file at the same time. This shouldn't happen
in normal operation, but an attacker could cause this, e.g. with
specially crafted network packets.
src/plugin.c: Assure rf->rf_next_read is initialized.

Otherwise read callbacks will be called twice on startup, causing all
sorts of trouble.
rrdtool and rrdcached plugins: Don't update the file if ...

... it is being created asynchronously.
collectd.conf(5): Improve documentation of CreateFilesAsync.
Merge branch 'ff/rrd-async'