[FEATURE] Support allocations to thousands of round members

[aerugo]

At the moment the platform does not manage handling thousands of members in a round.

• Pasting comma separated emails into the invite box does not work for more than about 150 at a time. (addressed separately in issue [FEATURE] Support inviting thousands of round members #1066)
• Adding and/or setting balance for is unreliable for more than a few hundred users and usually times out.

Setting or adding to balances should still be possible in-platform with thousands of users, so this needs to be fixed and tested. We should benchmark for the platform being able to handle a round of 10,000 members.

We need to optimize or rewrite the allocate and set mutations to be able to manage this in batch.
This may require writing some custom SQL to do this efficiently.