fix: Hopefully fix all tests being flaky

It seems that the varint encoding function would not work for
some integers as input. This should in theory fix this issue. Since
the SPK IDs are randomly between 0 and 2**32 - 1, it makes sense that
the tests fail only sometimes.

lib/src/omemo/sessionmanager.dart

@@ -177,15 +177,17 @@ class OmemoSessionManager {
     // Pick the correct SPK
     final device = await getDevice();
     OmemoKeyPair? spk;
-    if (kex.spkId == device.spkId) {
-      spk = device.spk;
-    } else if (kex.spkId == device.oldSpkId) {
-      spk = device.oldSpk;
-    } else {
+
+    await _lock.synchronized(() async {
+      if (kex.spkId == _device.spkId) {
+        spk = _device.spk;
+      } else if (kex.spkId == _device.oldSpkId) {
+        spk = _device.oldSpk;
+      }
+    });
+    if (spk == null) {
       throw UnknownSignedPrekeyException();
     }
-
-    assert(spk != null, 'The used SPK must be found');
     
     final kexResult = await x3dhFromInitialMessage(
       X3DHMessage(
@@ -198,7 +200,7 @@ class OmemoSessionManager {
       device.ik,
     );
     final ratchet = await OmemoDoubleRatchet.acceptNewSession(
-      spk,
+      spk!,
       OmemoPublicKey.fromBytes(kex.ik!, KeyPairType.ed25519),
       kexResult.sk,
       kexResult.ad,