feat: implement resp DEL command

[hderms]

Adds:

• the RESP command parsing for DEL

Notes:

• we will potentially need to rethink the relationship between a protocol and the underlying storage if we end up having Redis use both segcache and other datastructures, as del is supposed to work on a set of keys of any data type. One protocol would then need to be able to juggle multiple storage layers and know which keys belong in which storage layer, so it can delete the entry from the correct one (or just delete it from all of them, but we'd still need to deal with this fact if we implement COPY)
• del doesn't dedupe or verify uniqueness of keys. Not sure what Redis does