brontide+peer: use internal sync/pool to reduce allocations

This ensures that under medium to high load, we eliminate all
allocations once we arrive a steady state, re working memory.

brontide/conn.go

@@ -287,3 +287,10 @@ func (c *Conn) RemotePub() *btcec.PublicKey {
 func (c *Conn) LocalPub() *btcec.PublicKey {
 	return c.noise.localStatic.PubKey()
 }
+
+// ClearPendingSend drops references to the next header and body buffers and
+// returns any pooled buffers back to their respective pools so that the memory
+// can be reused.
+func (c *Conn) ClearPendingSend() {
+	c.noise.releaseBuffers()
+}