fix race cond in translate_test
fix race cond in translate_test

implements locked Reader method on test.TranslateFile. This race condition is
only present in tests, and is due to a replica referring directly to a primary
instead of through an http.TranslateStore as it would in production. This patch
requires the replica to obtain a lock while accessing the primary, and requires
test.Reopen to obtain a lock when swapping out the pilosa.TranslateStore.

I was able to reproduce the race by running: "go test
-run=TestTranslateFile_PrimaryTranslateStore -race -count=10", and could not
reproduce it after this patch.

Somewhat unrelated, I came across a "fatal error: fault" triggered by trying to
print an open pilosa.TranslateFile. This seems to be related to the mmapped
".data" field. I wrote the test to document the issue, but I don't think it's
easily fixable.

travisturner/pilosa