Writes for an initial index build aren't timestamped, so when we call recoverToStableTimestamp, the index data for any in-progress index builds won't be reverted so may contain references to records that no longer exist. Therefore we should wait for any running background index builds to complete before starting rollback, so that we won't try to call recoverToStableTimestamp in the middle of this index build.

SERVER-30911 will timestamp the catalog changes that represent the intent to build an index. After calling recoverToStableTimestamp, storage will build all ready: false indexes before returning.