RFR: 8283681: Improve ZonedDateTime offset handling [v3]

[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).

Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:

  Override ZoneOffset::normalized, cache ZoneOffset::getRules, revert change to add 2nd parameter to ZoneId::getOffset

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7957/files
  - new: https://git.openjdk.java.net/jdk/pull/7957/files/355f192a..a04589b7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7957&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7957&range=01-02

  Stats: 35 lines in 5 files changed: 29 ins; 0 del; 6 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