Move SMPP message data cache into its own class.
Collect submit_sm responses for multipart SMPP messages to avoid extra events.
Short-circuit multipart event processing on first failure.
Fix tyop.
Better comment about race condition.
Clearer part status check.
Fix tyop.
Use SETEX rather than SET and EXPIRE.
Merge branch 'feature/issue-818-cache-split-messages' into develop