RFR: 8257072: ZGC: Rename roots iterators [v4]

[Per Liden]

> Since we no longer have any STW roots (they are all concurrent now), we can shorten the names of the roots iterators to just ZRootsIterator and ZWeakRootsIterator.

Per Liden has updated the pull request incrementally with one additional commit since the last revision:

  Revert indentation change

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1429/files
  - new: https://git.openjdk.java.net/jdk/pull/1429/files/230f6fe5..133a6b08

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1429&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1429&range=02-03

  Stats: 17 lines in 5 files changed: 12 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1429.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1429/head:pull/1429

PR: https://git.openjdk.java.net/jdk/pull/1429