RFR: Shrink tlab to capacity [v2]

[Kelvin Nilsen]

> When a TLAB request exceeds the currently available memory within young-gen, the existing behavior is to reject the TLAB request outright. This is recognized as a failed allocation request, which triggers degenerated GC.
> 
> This change introduces code to reduce the likelihood that too-large TLAB requests will be issued, and when they are issued, it makes an effort to shrink the TLAB request in order to reduce the need for degenerated GC.
> 
> The impact is difficult to measure because this situation is fairly rare.  On one Extremem workload, the TLAB-shrinking code is exercised only once during a 16-minute run involving 500 concurrent GCs, a 45 GiB heap, and a 28 GiB young-gen size.  The change reduces the degenerated GCs from 6 to 5.
> 
> One reason that the remaining 5 degenerated GCs are not addressed by this change is that further work is required to handle a situation in which a requested TLAB is smaller than the available young-gen memory, but available memory is set aside in the evacuation reserve so cannot be provided to a mutator.  Future work will address this condition.

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

  Clarify recursive implementation of allocate_memory_under_lock
  
  (with a comment)

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/180/files
  - new: https://git.openjdk.org/shenandoah/pull/180/files/774e07a1..2d5da073

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=180&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=180&range=00-01

  Stats: 20 lines in 1 file changed: 17 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah/pull/180.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/180/head:pull/180

PR: https://git.openjdk.org/shenandoah/pull/180