Add support for @IdGeneratorType

[DavideD]

Closes #1906

This is PR #2476 with a few extra commits.
It's just a bit of refactoring that also removes some unused or wrongly included code (probably).

@dreab8, please, could you check these things and let me know if they make sense:

• I put together the check related to CockroachDB so that we can more easily find them when we are going to change that part
• I removed ReactiveAbstractSelectingDelegate: It doesn't seem to have any use
• I clean up the test cases a little, there were a couple of lines of code that weren't doing anything.

Regarding the tests there are a couple of things that I don't understand.

1. Why are we checking if the quey contains "update"? Because if we take a test as example, the query looks like this:

   select iav1_0.id,iav1_0.data,iav1_0.name,iav1_0.updateDate from IdentityAndValues iav1_0 where iav1_0.id=$1

   Is the intent to test that it contains the column updateDate or an update sql command? We should at least add a comment

2. there's a method called assertNumberOfOccurrenceInQueryNoSpace that I don't undestand what it does. Could you add a javadoc with an example of the queries? Maybe specify which databases we support actually run that check.
   Also the assertions looks like this:

   assertThat( actual ).as( "number of " + toCheck ).isEqualTo( expectedNumberOfOccurrences );

   Meaning that a message error looks like this org.opentest4j.AssertionFailedError: [number of data]

   I suppose it should be:

   assertThat( actual ).as( "Unexpecte number of '" + toCheck + "' in the query " + query ).isEqualTo( expectedNumberOfOccurrences );

I think we can merge this PR and close the issue for now (after a review of the extra commits). But let me know.

[dreab8]

Hi @DavideD I added a comment to assertNumberOfOccurrenceInQueryNoSpace hoping it clarifies the method role and also fixed the contains update assertions basically we were checking the wrong query (see also hibernate/hibernate-orm#10864).
Marco Belladelli explained that for update and insert we are using contains instead of startsWith because some delegates will prepend some other statements before the insert (e.g. DB2 will use select ... from new table(insert into ...)) or update.