RFR: 8276220: Reduce excessive allocations in DateTimeFormatter

[Claes Redestad]

Prompted by a request from Volkan Yazıcı I took a look at why DataTimeFormatters are much less efficient for some common patterns than custom formatters in apache-commons and log4j. This patch address some of that gap, without having looked at the third party implementations. 

When printing times:
- Avoid turning integral values into `String`s before appending them to the buffer 
- Specialize `appendFraction` for `NANO_OF_SECOND` to avoid use of `BigDecimal`

This means a speed-up and reduction in allocations when formatting almost any date or time pattern, and especially so when including sub-second parts (`S-SSSSSSSSS`).

Much of the remaining overhead can be traced to the need to create a `DateTimePrintContext` and adjusting `Instant`s into a `ZonedDateTime` internally. We could likely also win performance by specializing some common patterns.

Testing: tier1-3

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Commit messages:
 - 8276220: Reduce excessive allocations in DateTimeFormatter

Changes: https://git.openjdk.java.net/jdk/pull/6188/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6188&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276220
  Stats: 429 lines in 4 files changed: 407 ins; 10 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6188.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6188/head:pull/6188

PR: https://git.openjdk.java.net/jdk/pull/6188