- More tweaks to cx_oracle Decimal handling.

- More tweaks to cx_oracle Decimal handling.
"Ambiguous" numerics with no decimal place
are coerced to int at the connection handler
level.  The advantage here is that ints
come back as ints without SQLA type
objects being involved and without needless
conversion to Decimal first.

Unfortunately, some exotic subquery cases
can even see different types between
individual result rows, so the Numeric
handler, when instructed to return Decimal,
can't take full advantage of "native decimal"
mode and must run isinstance() on every value
to check if its Decimal already. Reopen of
[ticket:1840]