CLEANUP: cache: use read_u32/write_u32 to access the cache entry's hash

CLEANUP: cache: use read_u32/write_u32 to access the cache entry's hash

Enabling strict aliasing fails on the cache's hash which is a series of
20 bytes cast as u32. And in practice it could even fail on some archs
if the http_txn didn't guarantee the hash was properly aligned. Let's
use read_u32() to read the value and write_u32() to set it, this makes
sure the compiler emits the correct code to access these and knows about
the intentional aliasing.