build: remove lnd dep by including tlv varint impls

In this commit, we remove the circular-ish dep with `lnd` by removing
our dependency on the tlv package. We do this by copying over the
implementation of the varint encoding/decoding from the tlv package.

Author: Roasbeef

go.mod

@@ -6,6 +6,8 @@ require (
 	github.com/btcsuite/btclog v0.0.0-20170628155309-84c8d2346e9f
 	github.com/btcsuite/btcutil v0.0.0-20190425235716-9e5f4b9a998d
 	github.com/davecgh/go-spew v1.1.1
-	github.com/lightningnetwork/lnd v0.7.1-beta.0.20190807225126-ea77ff91c221
 	golang.org/x/crypto v0.0.0-20190211182817-74369b46fc67
+	golang.org/x/sys v0.0.0-20190209173611-3b5209105503 // indirect
 )
+
+go 1.13