introduce AR::ConnectionAdapters::PostgreSQL for sharing modules (with AR-JDBC)
... 'shared' OID, ArrayParser and Cast helpers, also re-arranged Column's dependencies
PostgreSQL's SchemaStatements seems a could candidate for re-use (with AR-JDBC)
move PostgreSQL's DatabaseStatements out of the PostgreSQLAdapter name-space
PostgreSQLAdapter::Utils seems to be only used from a single spot - quite redundant
Merge pull request #15088 from kares/shareable-modules

[postgres] remove (outer) sub-class declarations with nested modules/classes
Methods of these modules are internal

They should not be used on people application so they should not be
present on the API documentation.
:scissors: