write_graphite plugin: Actually check the return value of wg_send_message().

Assignment to "status" was missing.
write_graphite plugin: Don't call close() on invalid file descriptors.
write_graphite plugin: Don't call close() on invalid file descriptors.
src/plugin.c: Free the "data_sets" AVL tree.

This pops up every time I investigate memory leaks and it's annoying me.
Added new WriteQueueLengthLimit (drop values when bigger)

Signed-off-by: Florian Forster <octo@collectd.org>
fixed a bug in doc near new WriteQueueLengthLimit*

Signed-off-by: Florian Forster <octo@collectd.org>
Fixes for code quality

Signed-off-by: Florian Forster <octo@collectd.org>
Fixed a bug with WriteQueueLengthLimitHigh is null/uninitialized

Signed-off-by: Florian Forster <octo@collectd.org>
src/configfile.c: Rename options to "WriteQueueLimit{High,Low}".

The original names are a bit long.
src/plugin.c: Some fixes for write limits.

* Log an error once per second.
* Coding style fixes.
* Separate function for calculating drop probability.
Merge branch 'collectd-5.2' into collectd-5.3
Merge branch 'collectd-5.3'

Conflicts:
	src/plugin.c
Merge branch 'ym/limit_write_queue_length'