Use less iterations for KeyGenerator in tests

This commit improves performance of cookie tests:

Ruby  |   After  |   Before
----- | --------:| --------:
MRI   |    5.03s |     9.28s
JRuby |   25.45s |  1648.23s

Please note the improvement for JRuby.
Correctly detect mutation on serialized columns mapping to binary

Fixes #16701
[ci skip] Update 4_2_release notes
[ci skip] Fix typo of `collection_singular_ids=` method.
[ci skip] Fix typo of `collection=` method.
Merge pull request #16721 from yui-knk/fix/guides_association_basics

[ci skip] Fix typo of `collection=` method.
Merge pull request #16722 from yui-knk/fix/guides_association_basics2

[ci skip] Fix typo of `collection_singular_ids=` method.
Merge pull request #16702 from sgrif/sg-binary-serialized

Correctly detect mutation on serialized columns mapping to binary
Merge pull request #16707 from katmutua/master

update 4_2_release_notes
Merge pull request #16717 from splattael/keygenerator

Use less iterations for KeyGenerator in tests