RFR: 8270870: Simplify G1ServiceThread [v2]

[Kim Barrett]

> Please review this simplification of the internal control structure of
> G1ServiceThread, particularly around the entry and exit of monitor contexts.
> 
> In addition, the run_task logging now includes the delay between the
> scheduled time to run and the actual, as that seems like useful information
> to have sometimes.
> 
> I also changed the monitor rank from nonleaf to leaf.  There was no reason
> to use nonleaf, and leaf avoids questions about why nonleaf was used.
> 
> Testing:
> mach5 tier1-3.  This includes the gtest for G1ServiceThread.
> 
> Ran a test program with gc+task logging enabled and checked that the logging
> output looked okay.

Kim Barrett has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge branch 'master' into g1service_thread
 - improve assert
 - peek/pop -> front/remove_front
 - tidy run_service
 - log schedule delay
 - simplify task wait loop and monitor usage

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4868/files
  - new: https://git.openjdk.java.net/jdk/pull/4868/files/1a2e4e18..bd364d33

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4868&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4868&range=00-01

  Stats: 1793 lines in 122 files changed: 1224 ins; 241 del; 328 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4868.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4868/head:pull/4868

PR: https://git.openjdk.java.net/jdk/pull/4868