Use block reading utility method in `RedactingEventHashBlockStreamValidator`

The `RedactingEventHashBlockStreamValidator` class writes blocks to disk, then reads them back in using custom code. The reading of blocks from disk should be replaced with an existing utility method, `BlockStreamAccess.readBlocks`. Initial efforts to do this failed, so there is some issue to be figured out.

Start in RedactingEventHashBlockStreamValidator and compare its custom block-reading code with the BlockStreamAccess.readBlocks utility. Investigate why earlier replacement attempts failed, then verify that blocks written to disk are read and validated with the utility without changing behavior.

Written by the indexing model from the issue text.