out_pgsql: refactor plugin logic to follow new config maps and threads

The plugin was too old and required an update as follow:
* Use of the config map for the config options
* Decoder for the log events
* Use of the internal instances to avoid having a pool of connections

Also, some unit tests were added to make it easy to maintain the plugin

Signed-off-by: Jonathan Gonzalez V. <jonathan.abdiel@gmail.com>

Author: sxd