Split PostgresClient.java into several classes:

• reading Postgres configuration data

• Postgres connection pool

• database operations (get, update, save, delete, join, ...)

• helper functions like runSQLFile(...)

PostgresClient.java is the biggest and most complex rmb file because

it is 1,846 lines long
https://sonar.gbv.de/component_measures/metric/lines/list?id=org.folio%3Araml-module-builder
and has a complexity of 259
https://sonar.gbv.de/component_measures/metric/file_complexity/list?id=org.folio%3Araml-module-builder

Splitting this file makes usage and maintenance more easy because of separation of concerns (single responsibility principle).