RFR: 8291061: Improve thread safety of FileTime.toString and toInstant

[Сергей Цыпанов]

On Fri, 22 Jul 2022 07:34:19 GMT, Andrey Turbanov <aturbanov at openjdk.org> wrote:

> We need to read fields `instant`/`valueAsString` once, as to dodge/resolve the data race on reading lazily-initialized fields.

Marked as reviewed by stsypanov at github.com (no known OpenJDK username).

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PR: https://git.openjdk.org/jdk/pull/9608