fix(auth): delete SyncGatewaySession cookie when logging out

Delete the SyncGateway session cookie.
This fixes log in/log out for the time being.
Feature in progress to handle that automatically [#543](https://github.com/couchbase/couchbase-lite-ios/issues/543)

Deleting the cookie may sometimes produce the warning:

```
16:23:05.285‖ WARNING: CBL_Pusher[http://demo.mobile.couchbase.com/todolite]: Unable to save remote checkpoint: Error Domain=CBLHTTP Code=401 "401 unauthorized" UserInfo=0x7f9b3bd6a150 {NSURL=http://demo.mobile.couchbase.com/todolite/_local/acc396601fc0f2472b2521b06861dcf24dd9aa4a, NSLocalizedFailureReason=unauthorized, NSLocalizedDescription=401 unauthorized}
```