Do not cache query plan with non-parameterized constants.

[fredericDelaporte]

• Fixes NH-3673 - Closure variable values locked in from expressions in NHibernate LINQ provider #1330 (NH-3673)
• Obsoletes NH-2658 - Allow the IHqlGeneratorForMethod to decide how to build a query cache key #866 (NH-2658)
• Fixes NH-2500 - NH 3.0 Linq provider uses query parameters from first call in subsequent calls. #1363 (NH-2500)

This is a simpler alternate solution than #1542, and complete: no workaround is required for any case. This would obsolete #1540 by the way, as would do #1542.

Vs #1542, pros:

• Simpler.
• Would solve last NH-2500 - NH 3.0 Linq provider uses query parameters from first call in subsequent calls. #1363 trouble without the workaround.
• Would avoid retaining captured object instances.
• Would avoid bloating the cache with as many entries as constant values used with queries having non-parameterized constants, when an application use many different values for such constants.

Cons:

• Would never cache plan for queries having non-parameterized constants. If such queries are called many times with same non-parameterized constants values, their execution would be slightly slower. (But it should not be a significant performance hit in the case of a query going to DB, the db querying would still take most of the overall execution time.)

The "deluxe" solution would be to combine #1540 and #1542, defaulting to #1542 behavior (safer) with an option for enabling #1542 behavior (documented as potentially causing object instances caching and reuse). But I do not think this would be worth the additional code for supporting it.