The code that writes ZIL blocks uses dmu_tx_assign(TXG_WAIT) to assign a
transaction to a transaction group.
That seems to be logically incorrect as writing of the ZIL block does not
introduce any new dirty data.
Also, when there is a lot of dirty data, the call can introduce significant
delays into the ZIL commit path,
thus affecting all synchronous writes. Additionally, ARC throttling may affect
the ZIL writing.
We probably need a new mechanism similar to dmu_tx_create_assigned to assign
ZIL transactions.
(Ab)using TXG_WAITED does not seem to be sufficient.