RFR: JDK-8268352: Rename javadoc Messager class to JavadocLog

[Jonathan Gibbons]

Please review a simple cleanup, to rename the javadoc Messager class to the more explicit, less-confusing name of JavadocLog, since it is the javadoc subtype of the javac Log class.

The new name is in line with other javadoc subtypes of javac classes.

The change only affects the javadoc.tool package; it does not affect any doclet packages, which use the `Reporter` interface, provided by `JavadocLog`.

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Commit messages:
 - JDK-8268352: Rename javadoc Messager class to JavadocLog

Changes: https://git.openjdk.java.net/jdk/pull/4405/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4405&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268352
  Stats: 122 lines in 8 files changed: 0 ins; 0 del; 122 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4405.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4405/head:pull/4405

PR: https://git.openjdk.java.net/jdk/pull/4405