Do not create a global statsd "previous instance"

this basically reverts #887

at some point we might want to do some special handling of reloading
plugins and keeping their state intact, but that will need to be done at
a higher level, and in a way that is thread-safe for multiple input
plugins of the same type.

Unfortunately this is a rather large feature that will not have a quick
fix available for it.

fixes #1975
fixes #2102