RFR: 8255405: sun/net/ftp/imp/FtpClient uses SimpleDateFormat in not thread-safe manner [v3]

[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

Igor Ignatyev has updated the pull request incrementally with two additional commits since the last revision:

 - use UTC TZ for RFC3659_DATETIME_FORMAT
 - run TestFtpTimeValue with different user.timezone

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/867/files
  - new: https://git.openjdk.java.net/jdk/pull/867/files/a8e6d809..322e04f3

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=867&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=867&range=01-02

  Stats: 9 lines in 2 files changed: 4 ins; 1 del; 4 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