RFR: 8283681: Improve ZonedDateTime offset handling

[Claes Redestad]

Richard Startin prompted me to have a look at a case where java.time underperforms relative to joda time (https://twitter.com/richardstartin/status/1506975932271190017). 

It seems the java.time test of his suffer from heavy allocations due ZoneOffset::getRules allocating a new ZoneRules object every time and escape analysis failing to do the thing in his test. The patch here adds a simple specialization so that when creating ZonedDateTimes using a ZoneOffset we don't query the rules at all. This removes the risk of extra allocations and slightly speeds up ZonedDateTime creation for both ZoneOffset (+14%) and ZoneRegion (+5%) even when EA works like it should (the case in the here provided microbenchmark).

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Commit messages:
 - Fix names, typos
 - Avoid going through ZoneRules for ZoneOffsets

Changes: https://git.openjdk.java.net/jdk/pull/7957/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7957&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283681
  Stats: 126 lines in 5 files changed: 123 ins; 2 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7957.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7957/head:pull/7957

PR: https://git.openjdk.java.net/jdk/pull/7957