Awaiting broadcast of async transactions fails

[Cashmaney]

When trying to send an async transaction via Tendermint34Client it will return an error when waiting for broadcastTxAsync to return.

Can be triggered by calling

const result = await tmClient.broadcastTxAsync({tx: txBytes});

Which results in:

e: TypeError: Cannot read property 'decodeBroadcastTxSync' of undefined
(node:38652) UnhandledPromiseRejectionWarning: ReferenceError: result is not defined
    at run (C:\Users\Itzik\WebstormProjects\signingtest\main_sg.js:62:28)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
(node:38652) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1
)
(node:38652) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

Even though the broadcast itself is successful. This small PR fixes this issue

Tested with Secret Network Testnet running Tendermint version v0.34.14

[webmaster128]

Nice find! This affects pretty much all versions of CosmJS and can easily be reproduced in tests. It seels like this function was never used and never tested. Shame on us. It also shows that you should not use classes for namespacing in JavaScript. We don't write Java.

I'll write a patch that for 0.26.x and add the missing tests.

[webmaster128]

Fix released as part of 0.26.5