RFR: Add generational full gc support [v2]

[Kelvin Nilsen]

> In situations when Generational Shenandoah triggers full-gc, we now allow full gc to run.  The implementation of full gc preserves the distinction between young and old memory regions because premature promotion of young objects into old regions may result in inefficient subsequent old-gc efforts.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Remove extraneous debugging infrastructure

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Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/61/files
  - new: https://git.openjdk.java.net/shenandoah/pull/61/files/c93f9599..ac520ad8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=61&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=61&range=00-01

  Stats: 14 lines in 1 file changed: 0 ins; 14 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/61.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/61/head:pull/61

PR: https://git.openjdk.java.net/shenandoah/pull/61