expose ArJdbc.warn on ArJdbcModule
[mysql] remove deprecated structure_dump method
[postgres] remove deprecated structure_dump method
[mssql] remove unused (dead) code
no longer keep deprecated MissingFunctionalityHelper constant
avoid unnecessary connection-factory ruby 2 java conversion
add in some more README/CONTRIBUTING notes (e.g. around :git sources)
support JdbcAdapter.new in a  Rails core compatible way ...

- `def initialize(connection, logger = nil, pool = nil)` on 3.2 / 4.x
- `def initialize(connection, logger = nil)` on 2.3 - 3.1
using ENV_JAVA instead of the Java API is faster
remove deprecated H2 adapter method
mariadb driver should default to using mariadb adapter + no need for i qualifier
avoid @connection instance variable (unecessary java.sql.Connection wrapping)