updateCached should return (api.EpochTime, bool) to avoid apparent race condition

Description of the bug

Ultra tiny nit.

https://github.com/oasislabs/ekiden/blob/2cb41bfab18c7e34288e476b3a7a3f12fbb7fd47/go/epochtime/tendermint/tendermint.go#L80

https://github.com/oasislabs/ekiden/blob/2cb41bfab18c7e34288e476b3a7a3f12fbb7fd47/go/epochtime/tendermint/tendermint.go#L74

If updateCached were to return (api.EpochTime, bool) instead of just the bool, then we would not need the comment on L74 since the apparent data race does not occur. There is only one goroutine thread running worker() and everything should be fine in all practical concurrency models, but this is potential a future false positive if/when we get a data race analyzer, so we might as well avoid it by using a nicer method signature.

Steps to reproduce

1. Read the code.

Expected behavior
In all possible futures where we get a race condition detectors / code analyzers (tsan-like tool for golang), they will not complain about this code, even if the analyzers were not very smart. 😄

Environment

• OS:
• Versions of relevant software and tools:

Additional information

Read go/epochtime/tendermint/tendermint.go at the referenced updateCached and worker locations, then trace how updateCached's return value is used. Change the signature and its caller so the cached EpochTime is returned with the boolean, and verify that the comment about the apparent race is no longer needed.