RFR: 8270870: Simplify G1ServiceThread

[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.

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Commit messages:
 - improve assert
 - peek/pop -> front/remove_front
 - tidy run_service
 - log schedule delay
 - simplify task wait loop and monitor usage

Changes: https://git.openjdk.java.net/jdk/pull/4868/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4868&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270870
  Stats: 109 lines in 3 files changed: 20 ins; 36 del; 53 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