Wait until lightwalletd full sync goes all the way to the tip

[teor2345]

Motivation

lightwalletd doesn't reliably log when it reaches the chain tip.

In PR #4828, we added a workaround to the lightwalletd full sync test, which stops at a fixed height near the chain tip. But for tests like #4350, we need to wait until it has actually reached the tip.

Designs

Make the lightwalletd full sync, update sync, and send transaction tests wait until lightwalletd has reached the chain tip.

We could do this by checking that the Zebra JSON-RPC and lightwalletd gRPC heights match.