WIP: Machine.WatchInterfaces NotifyWatcher

This is a NotifyWatcher that monitors the machine's network interfaces, and reports the Ids of network interfaces that are modified.
This should catch Added, Removed, Enabled, Disabled

Notes:
1) The current PR is implemented as StringsWatcher as suggested before
2) Some tests with AddNetworkInterface are broken (WIP), because dynamic network interfaces were recently allowed
3) I have changed _id in networkInterfaceDoc from ObjectId to string because of initial stringsWatcher implementation