Add skip-empty-xacts option to test_decoding for use in the regression tests.

 Add skip-empty-xacts option to test_decoding for use in the regression tests.
 
 The regression tests for contrib/test_decoding regularly failed on
 postgres instances that were very slow. Either because the hardware
 itself was slow or because very expensive debugging options like
 CLOBBER_CACHE_ALWAYS were used.
 
 The reason they failed was just that some additional transactions were
 decoded. Analyze and vacuum, triggered by autovac.
 
 To fix just add a option to test_decoding to only display transactions
 in which a change was actually displayed. That's not pretty because it
 removes information from the tests; but better than constantly failing
 tests in very likely harmless ways.
 
 Backpatch to 9.4 where logical decoding was introduced.
 
 Discussion: 20140629142511.GA26930@awork2.anarazel.de