Extend the logical decoding output plugin API with stream methods.

 Extend the logical decoding output plugin API with stream methods.
 
 This adds seven methods to the output plugin API, adding support for
 streaming changes of large in-progress transactions.
 
 * stream_start
 * stream_stop
 * stream_abort
 * stream_commit
 * stream_change
 * stream_message
 * stream_truncate
 
 Most of this is a simple extension of the existing methods, with
 the semantic difference that the transaction (or subtransaction)
 is incomplete and may be aborted later (which is something the
 regular API does not really need to deal with).
 
 This also extends the 'test_decoding' plugin, implementing these
 new stream methods.
 
 The stream_start/start_stop are used to demarcate a chunk of changes
 streamed for a particular toplevel transaction.
 
 This commit simply adds these new APIs and the upcoming patch to "allow
 the streaming mode in ReorderBuffer" will use these APIs.
 
 Author: Tomas Vondra, Dilip Kumar, Amit Kapila
 Reviewed-by: Amit Kapila
 Tested-by: Neha Sharma and Mahendra Singh Thalor
 Discussion: https://postgr.es/m/688b0b7f-2f6c-d827-c27b-216a8e3ea700@2ndquadrant.com