plugin: Pass on read-plugins' contexts to the write threads.

Else, the write plugins will have wrong or missing interval information when
actually writing a data-set.

Signed-off-by: Florian Forster <octo@collectd.org>
src/plugin.c: Make the context cloning a bit more implicit.

The plugin_set_ctx() call has been moved into the plugin_write_dequeue()
function. Comments describing the unavailability of the context have
been updated.