Pass a correct session end height to NewSession (#1545)

With session rollover (#1536), we accept relays for older sessions. For
exaple, a node at block 101 accepts a relay for the session height 97.

There is a bug in the relay validation function `relay.Validate`. In the
example above, the function sees the following values:

```
ctx.BlockHeight() = 101
sessionBlockHeight = r.Proof.SessionBlockHeight = 97
sessionCtx.BlockHeight() = 97
```

and if the corresponding session is not cached, it passes `sessionCtx`
and `ctx` to `NewSession`. This may return a wrong session because the
second argument is supposed to be the end of the session, but in this
case it's not.

The proposed fix is if `ctx` is beyond the relay session, we get a new
context of the session end and pass it to `NewSession`.

x/pocketcore/types/service.go

@@ -92,8 +92,23 @@ func (r *Relay) Validate(ctx sdk.Ctx, posKeeper PosKeeper, appsKeeper AppsKeeper
 		if err != nil {
 			return sdk.ZeroInt(), sdk.ErrInternal(err.Error())
 		}
+
+		// With session rollover, the height of `ctx` may already be in the next
+		// session of the relay's session.  In such a case, we need to pass the
+		// correct context of the session end instead of `ctx`.
+		sessionEndHeight :=
+			sessionBlockHeight + posKeeper.BlocksPerSession(sessionCtx) - 1
+		var sesssionEndCtx sdk.Ctx
+		if ctx.BlockHeight() > sessionEndHeight {
+			if sesssionEndCtx, err = ctx.PrevCtx(sessionEndHeight); err != nil {
+				return sdk.ZeroInt(), sdk.ErrInternal(er.Error())
+			}
+		} else {
+			sesssionEndCtx = ctx
+		}
+
 		var er sdk.Error
-		session, er = NewSession(sessionCtx, ctx, posKeeper, header, hex.EncodeToString(bh), int(sessionNodeCount))
+		session, er = NewSession(sessionCtx, sesssionEndCtx, posKeeper, header, hex.EncodeToString(bh), int(sessionNodeCount))
 		if er != nil {
 			return sdk.ZeroInt(), er
 		}