Removing the form of checkForTransactionInThreadLocalStorage

[programmatix]

that does not use TransactionMarkerOwner. As this bypasses
a critical check on whether we are inside a blocking
transaction.

Two new tests in SDKTransactionsSaveIntegrationTests
(reactiveSaveInBlockingTransaction and blockingSaveInBlockingTransaction)
will fail without this, as the .save() does not realise
it's in a transaction.

Adding new tests, for .save() and for performing transactions
in reactor blocking threads.

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