mapper_configured(), after_configured(), and before_configured().

could encounter empty history, and where a column keyed to an alternate
attribute name would fail to track properly.  Fixes courtesy
Alex Fraser.

by mock and other __getattr__ impostors

or mapped instances into contexts where they are interpreted as
SQL bound parameters; a new exception is raised for this.
fixes #3321

refer mostly to the DDL object; this system is primarily useful
in that case, and not for built-in objects.  Reference that
the built-in case is not really viable right now. References #3442.

- new test for json col['x']['y']['z'] seems to fail pre PG 9.4,
fails on comparisons for non-compatible data instead of not matching
- no need to call SpecPredicate(db) directly in exclusion functions,
by using Predicate.as_predicate() the spec strings can have version
comparisons

"super" instead of hardcoding to "self.type" for the default return
value, the base Comparator was returning other_comparator.type.   It's
not clear what the rationale for this was, though in theory the
base Comparator should possibly even throw an exception if the two
types aren't the same (or of the same affinity?) .
- mysql.SET was broken on this because the bitwise version adds "0"
to the value to force an integer within column_expression, we are doing type_coerces here
now in any case so that there is no type ambiguity for this
operation