RFR: 8298264: Merge OffsetTableContigSpace into TenuredSpace

[Albert Mingkun Yang]

Small change of merging two classes to reduce inheritance chain length.

Test: hotspot_gc

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Commit messages:
 - s1-merge-tenured-space

Changes: https://git.openjdk.org/jdk/pull/11556/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11556&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298264
  Stats: 84 lines in 6 files changed: 7 ins; 56 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/11556.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11556/head:pull/11556

PR: https://git.openjdk.org/jdk/pull/11556