Change to use 0 instead of -1 as the default counter value

[ghbutton]

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[scottnonnenberg-signal]

This library mirrors the same library in java: https://github.com/signalapp/libsignal-protocol-java

If you can show that your intended changes reflect the java library behavior, then we would merge it.

[ghbutton]

You can see here that ChainKeys get created with an index of 0: https://github.com/signalapp/libsignal-protocol-java/blob/4f5e1ff299cea22cc75bb97249020a7da67b816d/java/src/main/java/org/whispersystems/libsignal/ratchet/RatchetingSession.java#L136

this gets passed into the Message key here: https://github.com/signalapp/libsignal-protocol-java/blob/4f5e1ff299cea22cc75bb97249020a7da67b816d/java/src/main/java/org/whispersystems/libsignal/ratchet/ChainKey.java#L51

Which sets the message key counter as well: https://github.com/signalapp/libsignal-protocol-java/blob/4f5e1ff299cea22cc75bb97249020a7da67b816d/java/src/main/java/org/whispersystems/libsignal/ratchet/MessageKeys.java#L18

[ghbutton]

I am less sure of this PR now, going to close and investigate to see if it is my error when sending -1