Implement Logical Decoding compatibility with Postgres 9.6. (#124)

* Implement Logical Decoding compatibility with Postgres 9.6.

Several function and column names have changed (xlog to wal) between
Postgres major versions 9.6 and 10, and also in Postgres 9.6 creating a
logical replication slot seems to always export a (new) snapshot.

In this PR we adjust pgcopydb behavior to rely on the logical replication
protocol CREATE_REPLICATION_SLOT command to export our snapshot when the
option --follow is used and the source server is Postgres 9.6.

* Add tests/follow-9.6 to the CI.

* Add support for Postgres 9.5 and 10.

Both needed special attention, in particular in the test suite.