RFR: 8273482: Remove "foreground work" concept from WorkGang [v2]

[Per Liden]

> JDK-8237354 introduced the concept of "foreground work" in WorkGang, as a special case for use by the HeapDumper. I propose that we remove this code, since this special use case can be solved without the need for the concept of "foreground work" in WorkGang.
> 
> As far as I can tell, there's no reason why it must be the VM thread that takes on the task of writing the header, iterating over roots, etc. So, in VM_HeapDumper::work(), instead of checking if the current thread is the VM thread we can just check if the worker_id is non-zero. That way, a single worker thread will take on the task of writing the header, iterating over roots, etc, and all other worker threads will continue to call worker_loop().
> 
> Testing:
>  - Passed Tier1-3
>  - Passed multiple runs of test/hotspot/jtreg/serviceability/dcmd/gc/
>  - Manually ran jcmd GC.heap_dump with various GCs enabled

Per Liden has updated the pull request incrementally with one additional commit since the last revision:

  Remove asserts

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5410/files
  - new: https://git.openjdk.java.net/jdk/pull/5410/files/74ab4a07..7a9abf0c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5410&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5410&range=00-01

  Stats: 4 lines in 3 files changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5410.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5410/head:pull/5410

PR: https://git.openjdk.java.net/jdk/pull/5410