Make the balancer gather storage statistics only for shards which have `maxSize` set

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 customers 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 (listed in order of preference):

• Only gather storage statistics for shards which have maxSize set (implemented by this ticket)
• Issue the listDatabases call in parallel against all shards so it doesn't take 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
• Add a parameter to listDatabases to allow it to return cached data size instead of every time {{fstat}}ing all the files