Use DeviceId type from libsignal

[gferon]

Throughout the codebase, we sometimes use primitive types like u32 where libsignal introduced custom types (mostly to improve readability, sometimes to introduce functionnality). DeviceId is one of the types we haven't started to use yet in our codebase.

For example, the sender_device field in the Metadata struct should make use of libsignal_protocol::DeviceId.