Add support for customizing transaction start

[BillyYccc]

Motivation:

Fixes #432

Conformance:

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[BillyYccc]

I wonder if we could use interface to replace enum for TransactionIsolationLevel and TransactionAccessMode so that different clients can extend to config specific options.

[BillyYccc]

Thanks for the explanation! @aguibert I will check how to get this works.

[gavinking]

Hi, what's the status of this one?

[vietj]

it should be rebased on master, reviewed and finished

[BillyYccc]

I will check how it would work this weekend, last time I stuck on the DB2 transactions since then there have been many changes in the client