Fix Verification getting stuck

Related: connect2x/trixnity-messenger/trixnity-messenger#613 (closed)

When two trixnity clients verify one another, verification fails.

Changes:

1. We wait until we have both their and our SasMacEventContent before signing and sending VerificationDone
2. Ensure we send and handle events in the correct order

Analysis:

1. When we receive SasMacEventContent, we send VerificationDone
2. When we sendAndHandle VerificationDone, we update our state to Done/WaitForDone
3. When we later sendAndHandle our own SasMacEventContent, our state is already at Done/WaitForDone
4. Handling SasMacEventContent requires our state to be at ComparisonByUser or WaitForMac

The process looks something like this:

1. handleVerificationStep(SasMacEventContent, isOurOwn=false) — Receive their event
2. __sendVerificationStepAndHandleIt(VerificationDoneEventContent)
3. ____handleVerificationStep(VerificationDoneEventContent, isOurOwn=true)
4. ______state = WaitForDone — update state
5. ____sendVerificationStep(VerificationDoneEventContent)
6. match() — send our event
7. __sendVerificationStepAndHandleIt(SasMacEventContent)
8. ____handleVerificationStep(SasMacEventContent, isOurOwn=true)
9. ______if (currentState is ComparisonByUser || currentState is WaitForMacs) — precondition fails