lnd: allow shutdown signal during IsSynced check

Prior to this commit, lnd could become unresponsive to shutdown signals
during the `IsSynced` check. Since the `IsSynced` check can occasionally
take a long time to complete, this could result in lnd failing to shut
down promptly.

docs/release-notes/release-notes-0.19.0.md

@@ -27,6 +27,10 @@
   cause a nil pointer dereference during the probing of a payment request that 
   does not contain a payment address.
 
+* [Fix a bug](https://github.com/lightningnetwork/lnd/pull/9137) that prevented
+  a graceful shutdown of LND during the main chain backend sync check in certain
+  cases.
+
 # New Features
 ## Functional Enhancements
 ## RPC Additions
@@ -75,4 +79,5 @@
 * CharlieZKSmith
 * Elle Mouton
 * Pins
+* Viktor Tigerström
 * Ziggie