fix: support PostgreSQL 9.6

[vlsi]

This PR adds PostgreSQL 9.6 regression test.

org.postgresql.jdbc.PgDatabaseMetaData#getIndexInfo is changed to return NULL AS ASC_OR_DESC for 9.6+.

9.6 does not have pg_am.amcanorder, thus getIndexInfo failed as the following:

testPartialIndexInfo(org.postgresql.test.jdbc2.DatabaseMetaDataTest)  Time elapsed: 0.055 sec  <<< ERROR!
org.postgresql.util.PSQLException: ERROR: column am.amcanorder does not exist
  Position: 427
    at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2299)
    at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:2015)
    at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:210)
    at org.postgresql.jdbc.PgStatement.execute(PgStatement.java:425)
    at org.postgresql.jdbc.PgStatement.executeWithFlags(PgStatement.java:322)
    at org.postgresql.jdbc.PgStatement.executeQuery(PgStatement.java:285)
    at org.postgresql.jdbc.PgDatabaseMetaData.getIndexInfo(PgDatabaseMetaData.java:2952)
    at org.postgresql.test.jdbc2.DatabaseMetaDataTest.testPartialIndexInfo(DatabaseMetaDataTest.java:605)

[codecov-io]

Current coverage is 70.47%

Merging #560 into master will increase coverage by +<.01%

1. 2 files (not in diff) in ...a/org/postgresql/ssl were modified. more
   • Misses -3
2. 6 files (not in diff) in .../org/postgresql/jdbc were modified. more
   • Misses +1
   • Hits -2
3. 2 files (not in diff) in .../org/postgresql/core were modified. more

@@             master       #560   diff @@
==========================================
  Files           146        146          
  Lines         15497      15495     -2   
  Methods           0          0          
  Messages          0          0          
  Branches       3062       3055     -7   
==========================================
  Hits          10920      10920          
+ Misses         4577       4575     -2   
  Partials          0          0          

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