Complete superseded background fetch handlers

[nvphungdev]

Summary

• complete a pending silent-push fetch handler with noData when a newer debounced push replaces it
• avoid overwriting the active background fetch completion handler when another fetch is already running
• keep duplicate background fetches from leaving iOS completion handlers unresolved

Validation

• git diff --check
• xcrun swiftc -parse sphinx/AppDelegate.swift sphinx/Crypter/SphinxOnionManager.swift

Closes #162.