onConfigure should also return PENDING if config processing is not completed

[mandarjog]

Here is an example of a whitelist plugin that uses externally managed white lists.

1. OnConfigure ( white_list_url ). --> url is valid so True
2. But URL is not fetched yet, so the plugin cannot process requests

options

1. Until fully initialized: the plugin just lets everything pass.
   This is left to the plugin to decide and code correctly.
2. listener is kept in "warming" not ready until all plugins are operational
   a. onConfigure return PENDING
   b. At a later point on iocompletion, plugin calls proxy_config_done({success, failure})

[mandarjog]

@PiotrSikora @jplevyak