[bug]: Shutdown signal has no effect when chain syncing issues occur

[ViktorTigerstrom]

When the chain syncing issue described in #9108 occurs, lnd will not shut down gracefully until the syncing process begins progressing again, even if a shutdown signal has been triggered.

The root cause is that this line in lnd.go is blocking:

lnd/lnd.go

Line 674 in edd9ade

synced, ts, err := activeChainControl.Wallet.IsSynced()

As a result, lnd will not shut down until this line has completed execution.

Regardless of what is causing the chain syncing issues, lnd should always be able to shut down. Therefore, we should modify the code so that this line is non-blocking, allowing lnd to proceed with the shutdown when a shutdown signal is triggered.