Complete duplicate background fetch handlers

[caydyan]

Summary

• complete duplicate silent-push fetch handlers immediately with .noData when another background fetch is already running
• keep the active background fetch completion handler intact so the in-flight task can finish normally

Validation

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

Closes #162.

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[caydyan]

Current-head validation rerun on 144c5432a69c233c0a6dfcacea3dcb8dc28b544a after correcting the PR base back to master:

• PR diff is now scoped to sphinx/Crypter/SphinxOnionManager.swift only (1 file changed, 1 insertion, 2 deletions).
• git diff --check origin/master...HEAD passed.
• xcrun swiftc -parse sphinx/Crypter/SphinxOnionManager.swift passed.

I checked the previously requested develop base, but that branch is stale and does not contain the background-fetch code touched here; using it made GitHub show a 485-file PR diff from unrelated upstream master changes. The code change itself remains the same single-file duplicate-completion fix.