This commit adds a caching system that will save packet data in a much
more compact form than the regular MeshPacket protobuf. It cannot be
worked with directly to the same degree (although the packet header is
available), but consumes *much* less memory, and as a result can be used
to temporarily store large numbers of packets.
Cached packets can be retrieved either by their (from, id) tuple, or by
their hash.
This cache is a pre-requisite for the upcoming packet replay feature.
