One of my pending work items for post-realm merge.
The lowest-level import task is no longer asynchronous, as we don't want
it to span multiple threads to allow easier interaction with realm.
Removing the `Task` spec simplifies a heap of usages.
Individual usages should decide whether they want to run the import
asynchronously, by either using an alternative override or spooling up a
thread themselves.
If the operation timed out on..
```csharp
throw new TimeoutException(@"Took too long to acquire lock");
```
..from an update thread, it would not restore the update context.
The next call would then fail on the assert that ensures a non-null
context in such cases.
Can add test coverage if required.