chore: Simplify cache entry reading and writing

chore: Simplify cache entry reading and writing

Cache entries are now fully read into memory before (de)compressing,
checksumming and parsing, instead of streaming data like before. While
this increases memory usage when working with large object files, it
also simplifies the code a lot. Another motivation for this change is
that cache entry data is not streamed from secondary storage anyway, and
it makes sense to keep the architecture simple and similar for primary
and secondary storage code paths.

The cache entry format has modified so that the checksum covers the
potentially compressed payload (plus the header), not the uncompressed
payload (plus the header) like before. The checksum is now also stored
in an uncompressed epilogue. Since the cache entry format has been
changed, the input hash has been changed as well.