tatanka/client: separate network parts from mesh protocol parts

[buck54321]

Separate the networking logic from the mesh protocol logic by splitting the old TankaClient into two parts. MeshConn handles connections to both tatanka nodes and peers. Mesh handles the client mesh protocol. Mesh is somewhat similar to core.dexConnection in its purpose.

Here's a taste of the separated APIs.

type TheConnectionPart interface {
	ConnectPeer(peerID tanka.PeerID) error
	RequestPeer(peerID tanka.PeerID, msg *msgjson.Message, thing any) (*mj.TankagramResult, error)
	Auth(tatankaID tanka.PeerID) error
	RequestMesh(msg *msgjson.Message, thing any, opts ...RequestOption) error
}

type TheMeshFunPart interface {
	ID() tanka.PeerID
	Next() <-chan any
	Broadcast(topic tanka.Topic, subject tanka.Subject, msgType mj.BroadcastMessageType, thing interface{}) error
	PostBond(bond *tanka.Bond) error
	ActiveBonds() ([]*tanka.Bond, error)
	SubscribeMarket(baseID, quoteID uint32) error
}

Mesh has its own database. That is where things like order info and bonds will be stored. I've implemented a bonds table to demonstrate.