Cap all computations based on untrusted data

[GallagherCommaJack]

for example:

• limit number of kdf iterations for out of order message delivery
• limit max message size
• patch serde_cbor to limit maximum memory allocation

I'm not sure how best to pick these limits, so for now I'm going to leave TODO's in the code that reference this issue.

(probably partial) list of places in the code where this needs to be fixed:

• read_cbor
• send_cbor
• serde_cbor internals?
• unsent message storage
• registration loop
• login loop

[GallagherCommaJack]

tagged as good first issue because I think it's not too hard but will end up giving a pretty thorough tour of the codebase

[mobile-bungalow]

• read_cbor
• send_cbor
• unsent message storage