Statistics on Write Queue length (to help on WriteQueueLimitHigh tuning)

I have a Collectd running in an overloaded server.

PR #640 (please merge it) prevents a thread bomb.
PR #280 (already merged in collectd-5.4.0) and PR #690 (recently created, please merge it) fix most problems.

However, I had no idea how to configure the `WriteQueueLimitHigh` and `WriteQueueLimitLow` parameters introduced with PR #280. So I wrote this patch to have statistics on Write Queue and improve those parameters.

Note : we have some internal statistics on the Read Queue from the network plugin. The Write Queue is an internal queue so I prefered not to use `network` (as `p` in the `h/p-pi/t-ti` notation) but `internal`. Some may think that `collectd` sounds better than `internal`. Choose the best.

`nb_values` comes from another (unmerged yet) patch and I missed that it was not in this one. Sorry.
I agree with the fact that we can reuse one, and I like `cache_size` too. Patch updated.