RFR: Encode phase for each generation separately

[William Kemper]

This change encodes the state/activity of each generation separately for the overall collector status. There are complementary changes in [my fork of the visualizer](https://github.com/earthling-amzn/shenandoah-visualizer/tree/genshen-visualizer). There is no explicit versioning of the protocol between the encoding in the collector and the decoding in the visualizer. The change in this PR is compatible with the `genshen-visualizer` branch in my fork, but is _not_ compatible with the [upstream visualizer](https://github.com/openjdk/shenandoah-visualizer). Should I open a PR in the visualizer? Should we add a versioning scheme to the prototcol and try to maintain compatibility with older versions?

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Commit messages:
 - Encode phase for each generation separately

Changes: https://git.openjdk.java.net/shenandoah/pull/46/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=46&range=00
  Stats: 80 lines in 2 files changed: 65 ins; 11 del; 4 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/46.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/46/head:pull/46

PR: https://git.openjdk.java.net/shenandoah/pull/46