allow to bypass driver loading completely by setting `config[:load_driver] = false`
only resolve adapter_spec when JdbcAdapter not sub-classed
... all of our supported adapters use standalong adapter classes (since 1.4)
it's best to not check for ConnectionPool class but chek if it quacks like a (AR) pool
need to keep setting config[:adapter_spec] to correctly recolve visitor for adapter: jdbc