start cleaning up some of our testing code - so we can "share"'em later
synchronize setting resolved visitor type
useful to warn when visitor type is being overwriten ...

this might currently only happen when two connections are made using `adapter: jdbc` for different databases (connection specs)
clean visitor type since out jdbc test use adapter: jdbc  with various DBs
[derby] support getting and setting transaction isolation on a connection

setting it using the JDBC method seems to avoid some probable locking :

SQLException: Column 'PARAM1' already exists. SQL State: X0Y68

(likely related to #497 as well)