Add support for extended transaction definitions

[mp911de]

We should accept the transaction name and a transaction log mark. Also, apply the isolation level when starting a transaction and resetting the isolation level to the previous one after tx cleanup.

See also:

• https://github.com/r2dbc/r2dbc-spi/pull/159
• https://docs.microsoft.com/en-us/sql/t-sql/language-elements/begin-transaction-transact-sql?view=sql-server-ver15