Reachability rework

* Only use reachability become reachable change status to trigger retry in replicator.

* When ChangeTracker gets a reachability error, stop the change tracker and report an error immediately and do not do retry.

* When CBLRemoteRequest gets a reachability error, report an error immediately and do not do retry.

* Change kCBLReplicatorOption_Network to kCBLReplicatorOption_AllowsCellularAccess.

* Add allowsCellularAccess option to CBLRemoteRequest and ChangeTracker.

* Expand CBLIsOfflineError to cover more error codes.

* Make vendor/WebSockets-Cocoa pointing to feature/allows_cellular_access_option branch.