RFR: 8253030: ZGC: Change ZMarkCompleteTimeout unit to microseconds

[Per Lidén]

The ZMarkCompleteTimeout is currently specified in milliseconds, and its value is 1 (i.e. as low as it can be). In
preparation for using a lower time out value than 1ms we should change the ZMarkCompleteTimeout unit to be
microseconds, instead of milliseconds.

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Commit messages:
 - Mark timeout

Changes: https://git.openjdk.java.net/jdk/pull/120/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=120&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253030
  Stats: 18 lines in 4 files changed: 6 ins; 0 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/120.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/120/head:pull/120

PR: https://git.openjdk.java.net/jdk/pull/120