Make disconnect async

Nobody has claimed this yet.

`wasmClientDisconnect` takes a function that's called when the disconnect has concluded, but `lnc-web` doesn't use it.

As a result, consumers end up needing to do something like this:

```js
lnc.disconnect()
logger.info('disconnecting after:', `payment_hash=${hash}`)
// wait for lnc to disconnect before releasing the mutex
await new Promise((resolve, reject) => {
let counter = 0
const interval = setInterval(() => {
if (lnc.isConnected) {
if (counter++ > 100) {
logger.error('failed to disconnect from lnc')
clearInterval(interval)
reject(new Error('failed to disconnect from lnc'))
}
return
}
clearInterval(interval)
resolve()
})
}, 50)
```

Happy to create a PR myself to the effect of

```js
async disconnect() {
return await new Promise(resolve => this.wasm.wasmClientDisconnect(resolve);
}
```

... if it makes sense

Start by locating the lnc-web disconnect() entry point and its wasmClientDisconnect callback. Make disconnect await completion and expose that completion to consumers, then verify callers can await it instead of polling lnc.isConnected.

Written by the indexing model from the issue text.