Calibrate the default magma_mem_quota_ratio

Calibrate the default magma_mem_quota_ratio (IIRC 10% currently). We need to consider that workloads we will be recommending to use Magma for initially, and for those workloads what's a sensible default value for the mem_quota_ratio. If that ends up being a high number such that there would be overlap with the CheckpointManager quota (whose default also currently being decided) then we probably need to reduce checkpoint_memory_ratio such that the sum of checkpoint_memory_ratio and magma_mem_quota_ratio is less than 100% - or possibly less than high watermark (85%) - of bucket quota. This is a task which likely requires input from a number of stakeholders - KV-Engine, Magma and Perf team, possibly along with PM when selecting the initial workloads we want to focus on.