Implemented combineLatest semantics

[jmhofer]

... at least if I understand them correctly...

This is an attempt at resolving #29

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[benjchristensen]

This looks good and fixes the behavior. Thank you @jmhofer for first finding this issue and then tackling the solution as I imagine it wasn't fun code to dig into.

The only thing I would want to look at changing (and it originates from my original code that you were modifying) is try to move away from blocking/synchronization towards non-blocking/atomics if possible to reduce contention while still retaining the atomicity required for the contract. But that is an optimization that may or may not warrant the effort and certainly does not need to hold up accepting this code as is to fix the fundamental problem.