Client: Consider using `bytes` or `Arc<Vec<u8>>` for DB to get rid of allocations.

[tomusdrw]

[rphmeier]

May be convenient for the overlayed changes, but reading from a trie or paritydb will basically always require allocations.

We can then reallocate again pre-emptively to put Bytes or Arc<Vec<u8>> into a cache, but I am not convinced that we can predict access patterns easily enough to have a useful cache.

[gavofyork]

closing for now pending further consideration