Correct unit tests and races

[billf]

• the "too late" go routine did not finish before the end of the function. correct this invalid usage by signalling "too late" using a channel close.
• the read and writes of the 'ok' boolean failed race detection. avoid ordering issues by using sync/atomic.
• the counters in the examples also suffered from ordering issues. use an atomic for stronger ordering.

This package now passes go test -race.

[billf]

friendly ping to @andres-erbsen

[jkanywhere]

hi @andres-erbsen, is there something we can do to assist merging this request?