ServiceEntryPointImpl::Sessions contains a SyncToken which is used to safely access Sessions::ByClientMap. ServiceEntryPointImpl::startSession races with other functions that modify the map:

1. startSession creates a Sessions::iterator.
2. A code block is started and a SyncToken is created (and therefore _sessions is locked).
3. A SessionWorkflow is created and inserted, and the iterator is set.
4. The code block ends, the SyncToken is destroyed, and _sessions is unlocked.
5. The workflow is started by retrieving from SessionWorkflow from the iterator.

If an operation occurs on _sessions between steps 4. and 5., then the iterator may be invalidated.