refactor(p2p): JSON encoding to use protobuf instead

[LePremierHomme]

Closing opendexnetwork/opendex.network#34.

Changes:

• p2p packets' checksum to utilize their protobuf encoding instead of generating a JSON string.
• SessionInit p2p packet fields needed for signing to be encoded via the same protobuf's scheme of the entire packet, instead of JSON. Using a subset of the fields isn't a problem with protobuf - the used fields will be encoded while the remaining ones won't exist on the binary representation.

BREAKING CHANGE: JSON encoding to use protobuf instead

[maybeast]

As this is a breaking change can we bump the minCompatibleVersion in the package.json so that newer nodes will reject p2p connections with older versions.

[LePremierHomme]

So should I bump package.json's version to 1.2.5 (and assign it to minCompatibleVersion as well)?

[maybeast]

So should I bump package.json's version to 1.2.5 (and assign it to minCompatibleVersion as well)?

Good point. We'll need to bump minCompatibleVersion when we do the next release. I'll do it.

We can leave it out of this PR - sorry for the confusion. I'll do some manual testing on this branch in the coming days. Think we can get this in. Thanks.