RFR: 8318296: Move Space::initialize to ContiguousSpace

[Albert Mingkun Yang]

Simple refactoring of moving methods to subclass. (`clear` is also moved, as it is called inside `initialize`.)

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Commit messages:
 - merge-subklass-space

Changes: https://git.openjdk.org/jdk/pull/16219/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16219&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318296
  Stats: 51 lines in 2 files changed: 17 ins; 29 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/16219.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16219/head:pull/16219

PR: https://git.openjdk.org/jdk/pull/16219