OF-2341 / OF-2342: Process S2S 'bounces' async to prevent deadlocks
When a stanza addressed to a remote domain cannot be processed, it is 'bounced'. Processing of these bounces has been a factor in at least two recent deadlocks on the Ignite domain.
This commit attempts to alleviate the problem by making the processing of the 'bounce' be asynchronous to the process that leads up to sending to original stanza (that was bounced). This decoupling allows locks/mutexes kept by the 'supplying' code flow to be released, before the 'bounce' is being processed (which arguably is a completely new stanza which needs not be processed under the original locks/mutexes anyway).
