distsql: add memory accounting to mergeJoiner

[asubiotto]

The mergeJoiner accumulates rows with the help of two streamGroupAccumulator. These streamGroupAccumulators copy rows and use a slice that refers to these EncDatumRows. @jordanlewis suggested adding some memory accounting to EncDatumRowAlloc which would be useful in a lot of other places that this is used.

Bonus: add the max allocated memory to the visual query plan stats. Refer to a40b96b for an example

cc @yuzefovich