Optimize the sharding balancer's cluster statistics gathering

The sharding balancer currently issues listDatabases against every single shard in order to access the totalSize value. This value is used for ensuring that a shard's storage maxSize is not exceeded for users which have that value set.

The listDatabases call is quite heavy, especially for nodes with large number of databases/collections since it will fstat every single file under the instance.

There are a number of optimizations we can make in order to make this statistics gathering less expensive in the presence of maxSize (listed in order of preference):

• Add a parameter to listDatabases to allow it to return cached data size instead of every time {{fstat}}ing all the files
• Issue the listDatabases call in parallel against all shards so different shards' execution overlaps
• Cache the per-shard statistics so that they are not collected on every single round/moveChunk invocation
• Collect the per-shard statistics asynchronously so that multiple concurrent moveChunk requests can benefit