RFR: Cleanup NumberSeq additions

[Aleksey Shipilev]

I have been looking into how to decrease our upstream exposure for `NumberSeq` changes, and it seems to be not solvable in an easy way. Normally, I would add `AbsSeq::add(AbsSeq& other)` methods and get to that from `HdrSeq::add(HdrSeq& other)`, and that almost works. The remaining bit is figuring out the decaying average/variance thingie. 

I spent an hour trying to come up with easily provable equations for _combining_ the decaying average/variance. The decaying average is somewhat easy, although there are boundary problems (decaying average treats 0-th element without adjustments, which requires remembering the `_last` element to compensate). Decaying variance looks remarkably scary in the few of my approaches.

If we enter with current code upstream, it would be the uphill battle to implemented decays and prove they work. So instead, I took the alternative approach of doing everything inside Shenandoah code, except the decays. This reverts `numberSeq.*` to upstream state.

Additional testing:
 - [ ] Linux x86_64 fastdebug `hotspot_gc_shenandoah`
 - [ ] Linux AArch64 fastdebug `hotspot_gc_shenandoah`
 - [ ] GitFarm

-------------

Commit messages:
 - Fix
 - Something else works
 - Work
 - Work

Changes: https://git.openjdk.org/shenandoah/pull/268/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=268&range=00
  Stats: 133 lines in 6 files changed: 26 ins; 63 del; 44 mod
  Patch: https://git.openjdk.org/shenandoah/pull/268.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/268/head:pull/268

PR: https://git.openjdk.org/shenandoah/pull/268