Make all object types have the same size limits

[ghost]

The size limits of objects currently vary depending on their type. This makes it harder to create new versions of old object types as for instance a pubkey longer than 440 bytes will not be relayed. So I think the only limit should be the limit of 2**18 bytes.

[PeterSurda]

I can review the code but I need to discuss with other people if the limit removal is a good idea.

[ghost]

This should not be changed unless ACKs are simultaneously limited to only be of object type msg. But it might make sense to document the current behavior.

For reference these are the current limits as implemented by PyBitmessage:

getpubkey (0): 42 <= size <= 200
pubkey (1): 146 <= size <= 440
msg (2): size <= 2**18
broadcast (3): 180 <= size <= 2**18 and version >= 2
anything else: size <= 2**18