RFR: 8255405: sun/net/ftp/imp/FtpClient uses SimpleDateFormat in not thread-safe manner

[Igor Ignatyev]

Hi all,

could you please review this small and trivial fix?

`sun/net/ftp/imp/FtpClient::dateFormats` is an array of `SimpleDateFormat` which are shared among all instances of `FtpClient`. the fact that `SimpleDateFormat` isn't thread-safe renders`FtpClient` to be non-thread-safe as well. the patch makes the only usage of `dateFormats` array, `parseRfc3659TimeValue` method, `synchronized`.

the problem was reported in #776

Thanks,
-- Igor

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Commit messages:
 - 8255405: sun/net/ftp/imp/FtpClient uses SimpleDateFormat in not thread-safe manner

Changes: https://git.openjdk.java.net/jdk/pull/867/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=867&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255405
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/867.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/867/head:pull/867

PR: https://git.openjdk.java.net/jdk/pull/867