src/plugin.[ch]: Implement `plugin_dispatch_values_async'.

src/plugin.[ch]: Implement `plugin_dispatch_values_async'.

This function differs from `plugin_dispatch_values' in that it will add
the value_list_t to a queue rather than calling the write functions
right away. This as at least two advantages:

  - The _async function will often return faster, since no file
    operation is done.

  - The ``read thread'' and the ``write thread'' are not the same
    thread. This makes it much easier for plugins that provide both,
    `read' and `write' callbacks.