SyncSession.resume/pause crash after receiving remote update

[cmelchior]

It looks like the SyncSessionImpl.nativePointer is somehow being outdated and crashes the app when being used (e.g. when calling pause/resume).

This is reproducible by having two apps communicating. After Device A sends changes to Device B, Device B will crash when interacting with the SyncSession pointer.

[cmelchior]

The root problem was due to a pointer going out of date. The sequence was as follows:

1. Device A: Open Flexible Sync with initialSubscriptions. This would trigger the creation of the SyncedContext which stored a copy of the current dbPointer.
2. Device B writes a change
3. Device A receives this change, integrates it, updates the Realm pointer and closes the old one
4. Device A access the SyncSession. This tried to lazily create the session pointer using the, now closed, dbPointer.
5. Device A would crash with a SEGFAULT.

Creating both Subscriptions and SyncSession immediately when accessed the first time fixes the problem.