Enhance "raise" strategy to include "raise_on_sql" option

Enhance "raise" strategy to include "raise_on_sql" option

The "raise_on_sql" option differentiates from "raise" in that
firing a lazy loader is OK as long as it does a simple
get from identity map.   Whereas "raise" is more useful
for the case that objects are to be detached.

As part of this, refactors the strategy initiation logic
a bit so that a LoaderStrategy itself knows what "key" was used
to create it, thus allowing variants of a single strategy
based on what the "lazy" argument is.  To achieve this we
have to also get rid of _get_strategy_by_cls().

Everything here is internal with the one exception of an apparently
undocumented, but not underscored, "strategy_class" key
on relationship().   Though it's not clear what
"strategy_class" accomplishes; at this point the strategy
system is extensible using Property.strategy_for().

Fixes: #3812
Change-Id: I812ad878ea5cf764e15f6f71cb39eee78a645d88