Transform wal2json JSON messages into SQL queries. (#48)

* WIP: infrastructure to parse a JSON message in our internal structs.

An internal representation for the JSON message then allows to be smart
about multi-values INSERT statements or switching to COPY, and issuing a
single DELETE statement for many values too, etc.

At the moment only the infrastructure to support that is coded, we always
fill-in single tuple per statement.

* Actually use the "include-timestamp" wal2json option.

* First pass at producing the SQL queries from the JSON messages.

* Add some testing coverage.

* Fix an error spotted in the gcc output on debian.

* Implement UPDATE and DELETE output in SQL.

* Review an Assert() and make it a first-class error reporting.

* Update docs.