factor single-table reflection wrappers into common mixin

factor single-table reflection wrappers into common mixin

Introduced _BackendsMultiReflection mixin in engine/default.py
that provides the get_columns(), get_pk_constraint(),
get_foreign_keys(), get_indexes(), get_unique_constraints(),
get_check_constraints(), get_table_comment(), and
get_table_options() single-table methods, each delegating to
the corresponding get_multi_* method with
filter_names=[table_name].

PostgreSQL, Oracle, and MSSQL dialects now inherit from this
mixin instead of duplicating the wrapper pattern.  Oracle
retains its _value_or_raise() override which applies
normalize_name() for case-folding.  MSSQL overrides
get_unique_constraints(), get_check_constraints(), and
get_table_options() with NotImplementedError since it has no
native get_multi_* for those yet.

Change-Id: If68bbf94b3956348fc7ae8179ff093d63dcdfbe2