The new command preventWritesForInsufficientDiskSpace will be used to block per-shard user writes for example when we saturate the disk. Since in the future we could want to use the same command for other reasons, it makes more sense for command and all related classes (such as the persisted document and the related block reason) to be more generic. We want to rename the command to BlockShardLevelUserWrites.

We should also rename the already existing blockUserWritesMode command to BlockClusterLevelUserWrites.