removing Mimer SQL support - was outdated at received no attention for a long-time
... (reviewed) support for Mimer is best to handle in a separate AR-JDBC extension gem
`prefetch_primary_key?` is available all the way down to AR 2.3 - no need for hacks
removing (no longer used) QuotedPrimaryKeyExtension - usable only with Oracle/Mimer
deprecating DB2/AS400 support in favor of the aasome standalone gem (closing #524)
by @pierrickrouxel at https://github.com/pierrickrouxel/activerecord-jdbcas400-adapter
align AR constant retrieving convention method name (to `getJdbcConnection`)
avoid checking for removed ArJdbc (nested) modules + do not use deprecated method
avoid the renamed (deprecated) SerializedAttributesHelper constant
make sure extension discovery only uses one of the possible multiple installed gems
introduce a (native) with_meta_data_from_data_source_if_any helper
... to make sure we handle the lookup/connection failures correctly
use RubyHash's fastARef when retrieving values - potentially count for (future) nulls