Remove unnecessary reification from Kotlin R2DBC extensions

[astiob]

This allows calling the extensions from generic code that has erased types, for example:

class CustomRepository<T>(val type: Class<T>) {
    @Autowired
    lateinit var db: R2dbcEntityTemplate

    // Fails to compile currently because usingAndAwait requires a reified T
    suspend fun create(obj: T) = db.insert(type).usingAndAwait(obj)
}

I’m not sure what to do about tests. I could add new tests with erased types, but they would necessarily duplicate the existing tests that have reified types—and especially in ReactiveSelectOperationExtensionsUnitTests, that’s quite a lot of code; and the new tests would be verifying the same runtime behaviour as the old tests (as this change only affects compilability), so they’d become redundant. Alternatively, the existing tests could be tweaked to ensure the types are erased; or the ability to use erased types could go completely untested.

I’m also not sure whether this is an ABI-compatible change. If not, I assume it will have to wait until a major-enough release where ABI breaks are allowed.

[astiob]

Er, to clarify about ABI: the reified versions are inline, so any code compiled against the old version is inevitably compatible with the new version, too. But I’m guessing that code compiled against the new version will be incompatible with older Spring binaries at runtime because it will try to call methods that don’t exist, so this likely has to wait until a release like 3.1.

Alternatively, the non-reified versions could continue to be marked inline, which would avoid all ABI issues entirely; but as far as I see, Spring’s existing non-reified Kotlin coroutine extensions appear to be never marked inline.

[schauder]

Thanks, that's merged and ported back to 3.0