Compiled query APIs has generically-typed parameters, but delegates to param object[]

[roji]

Our compiled query API surface is nice and generic, accepting up to 16 generic parameters (e.g. CompileAsyncQuery). However, these generic methods seem to call into non-generic methods which accept params object[], so that an array parameter is allocated and all parameter values are boxed.

Actually flowing generic parameters all the way into QueryContext (and from there into a generic DbParameter, if we ever get one), is likely to require quite a bit of work. In the meantime we can at least avoid the array allocation on every execution.