[write_mgr] Write Manager should determine have limit total size of instance queues

[jeromefroe]

Currently, when the write manager creates a new instance queue, each queue is created with the same size, regardless of how many instances there are. However, this can lead to unbounded memory usage as the number of instances increases. Instead of being given a constant queue size upon configuration, the write manager should be given a total queue size target and should try to size the instance queues such that the total size of all the queues is less than the target total size.