This commit makes the call to forwardBatch after locking
in Adds synchronous. This ensures that circuits for any Add
packets are added to the switch in the same order that they
are prescribed in the channel state. Though it is very unlikely
this case would arise, it may happen under more greater loads.
In addition, this also makes some trivial optimizations wrt. to
not spawning unnecessary goroutines if no settle/fail packets
are locked in.
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