normalize execute style for events, 2.0

normalize execute style for events, 2.0

The _execute_20 and exec_driver_sql methods should wrap
up the parameters so that they represent the single list / single
dictionary style of invocation into the legacy methods.  then
the before_ after_ execute event handlers should be receiving
the parameter dictionary as a single dictionary.   this requires
that we break out distill_params to work differently if event
handlers are present.

additionally, add deprecation warnings for old argument passing
styles.

Change-Id: I97cb4d06adfcc6b889f10d01cc7775925cffb116