pgsql: Add skip-empty-xacts option to test_decoding for use in the regr

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

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/cb3d2df90e240680438cb612edcd1598579f4d5c

Modified Files
--------------
.../test_decoding/expected/concurrent_ddl_dml.out  |  124 +++++---------------
contrib/test_decoding/expected/ddl.out             |   80 ++++---------
.../test_decoding/expected/decoding_in_xact.out    |    8 +-
contrib/test_decoding/expected/permissions.out     |    6 +-
contrib/test_decoding/expected/prepared.out        |   12 +-
contrib/test_decoding/expected/rewrite.out         |   30 +----
contrib/test_decoding/expected/toast.out           |   18 +--
.../test_decoding/specs/concurrent_ddl_dml.spec    |    2 +-
contrib/test_decoding/sql/ddl.sql                  |   30 ++---
contrib/test_decoding/sql/decoding_in_xact.sql     |    4 +-
contrib/test_decoding/sql/permissions.sql          |    6 +-
contrib/test_decoding/sql/prepared.sql             |    2 +-
contrib/test_decoding/sql/rewrite.sql              |    4 +-
contrib/test_decoding/sql/toast.sql                |    2 +-
contrib/test_decoding/test_decoding.c              |   45 ++++++-
15 files changed, 130 insertions(+), 243 deletions(-)