RFR: JDK-8254627: Cleanup {Abstract, Single, Split}IndexWriter classes [v2]

[Jonathan Gibbons]

> Inspired by other recent work to cleanup index generation, this change is primarily about cleanup for
> `AbstractIndexWriter`, `SingleIndexWriter`, `SplitIndexWriter` to reduce code duplication and to reduce the 3 class to
> just 1 that can handle either single index files or split index files.  As part of this cleanup, some anomalies were
> uncovered in the naming of annotation types, enum constants and records, which have also been addressed.
> The changes involved moving/merging code from `SingleIndexWriter` and `SplitIndexWriter` in to `AbstractIndexWriter`,
> which is now no longer abstract and is thus renamed to just `IndexWriter`. Regrettably, the rename is not properly
> tracked.

Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  minor review feedback

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/621/files
  - new: https://git.openjdk.java.net/jdk/pull/621/files/f0150f5a..0af37572

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=621&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=621&range=00-01

  Stats: 16 lines in 3 files changed: 2 ins; 5 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/621.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/621/head:pull/621

PR: https://git.openjdk.java.net/jdk/pull/621