Add apply_count and commit_count to status. Reset the apply_* and commit_* on read

Trying to graph wsrep_{apply|commit}_{oooe,oool,window} was impossible as the entered_ is invisible. Depending on flush status between graphing events isn't really useful in trying to graph things.

This patch exposes the entered_ monitoring variable as count (is there a better name) and resets this and all the other upon read.

If acceptable I'll update the documentation too.