Add support for dynamic magma_mem_quota_ratio

At low resident ratios, the fixed 10% quota for magma is insufficient to keep the hot index blocks in the magma blockcache. For improving on-disk performance, users may want to dynamically change the quota to tune for optimal performance.

For workloads such as random workload, it is optimal to use the memory to cache magma index blocks instead of using the bulk of memory to cache documents as we could reduce the number of I/O required per reading significantly by caching index blocks.