RFR: 8257072: ZGC: Rename roots iterators

[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.

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Commit messages:
 - 8257072: ZGC: Rename roots iterators

Changes: https://git.openjdk.java.net/jdk/pull/1429/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1429&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257072
  Stats: 56 lines in 8 files changed: 0 ins; 13 del; 43 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