SMR scheduler task tracking unreliable

The SmrScheduler inner class in MapServer has a multi-level data structure to track outstanding SMRs. However, the level are not sufficient to reliably track all tasks. At the top level are subscriber RLOCs, followed by EIDs. However, (original) source EIDs are also needed for uniquely identifying requests, and they are missing. As a result it's impossible to retrieve and cancel the right task.