Review Magma's default setting for magma_min_value_block_size_threshold

We added the setting for Elixir deployments (multi tenant, per bucket quota 100MB) so that one tenant doesn't take down entire process or consumes memory much more than its bucket quota. But since Elixir was not done and Magma's min bucket quota is 1GB, we can review the 64KB limit and decide on a new reasonable safe limit.