Set ICE candidate pool size option

So we can start candidate gathering in advance

src/MatrixClientPeg.ts

@@ -279,6 +279,10 @@ class _MatrixClientPeg implements IMatrixClientPeg {
             timelineSupport: true,
             forceTURN: !SettingsStore.getValue('webRtcAllowPeerToPeer'),
             fallbackICEServerAllowed: !!SettingsStore.getValue('fallbackICEServerAllowed'),
+            // Gather up to 20 ICE candidates when a call arrives: this should be more than we'd
+            // ever normally need, so effectively this should make all the gathering happen when
+            // the call arrives.
+            iceCandidatePoolSize: 20,
             verificationMethods: [
                 verificationMethods.SAS,
                 SHOW_QR_CODE_METHOD,