Optional query extensions now use TLV instead of a custom format.
Flags are encoded as varint instead of bytes as originally proposed. With the current proposal they will all fit on a single byte, but will be
much easier to extends this way.

We add one new class for each TLV type, with specific TLV types, and encapsulate codecs.

TLVs that extend regular LN messages can be represented as a TlvStream and not an Option[TlvStream] since we don't need
to explicitely terminate the stream (either by preprending its length or using a specific terminator) as we do in Onion TLVs.

No TLVs simply means that the TLV stream is empty.

Checksums in ReplyChannelRange now have the same encoding as short channel ids and timestamps: one byte for
the encoding type (uncompressed or zlib) followed by encoded data.

…al-tlv2

If a have a TLV stream of type MyTLV which is a subtype of TLV, and MyTLV1 and MYTLV2 are both
subtypes of MyTLV then we can use stream.get[MyTLV1] to get the TLV record of type MYTLV1 (if any)
in our TLV stream.

Checksums are just transmitted as a raw array, with optional compression as it would be useless here.

We will use them on mainnet as soon as lightning/bolts#557 has been merged.

…al-tlv2

We remove the ugly and inefficient zipWithIndex we had before