Limit total memory utilization for bulk index builds

A bulk index build of multiple indexes on a collection, for example using createIndexes, or during an initial sync, will use up to 100 MB of memory per index, or up to 6400 MB of memory total (given the limit of 64 indexes per collection). This can lead to out-of-memory on smaller machines. We should limit the total memory used on smaller machines by reducing the amount of memory used per index build, and/or limiting the number of simultaneous index builds.