Fix assertion on loading AOF with timed out script. (#8284)

If AOF file contains a long Lua script that timed out, then the `evalCommand` calls
`blockingOperationEnds` which sets `server.blocked_last_cron` to 0. later on,
the AOF `whileBlockedCron` function asserts that this value is not 0.

The fix allows nesting call to `blockingOperationStarts` and `blockingOperationEnds`.

The issue was first introduce in this commit: 9ef8d2f67 (Redis 6.2 RC1)