HHH-16726 adjust readability condition for strict read-write cache items according isolation level

[pb00067]

1. items are always readable from 2ndLevelCache if we accept phantom reads
2. items are readable when cached timestamp is equal to current session timestamp (trust items which were cached by ourself)

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[beikov]

Hi and thanks for creating a pull request for this issue. Please note that we don't merge improvements on maintenance branches, only bug fixes, so if you want to contribute this, please create a PR against the main branch.