RFR: 8254758: Change G1ServiceThread to be task based

[Stefan Johansson]

Please review this enhancement to make `G1ServiceThread` task based. This thread has evolved from having a single
responsibility to do more things. And there are plans to add more to it. To make this easier and to allow different
tasks to run at different intervals this change adds a simple task mechanism to the service thread.

The idea is that the service thread keeps a list of `G1ServiceTask`-objects that are ordered by when they should
execute. When no tasks are ready to be run the service thread check how far into the future the next task is scheduled
and sleeps/waits for that duration. If a new task is added while the thread is waiting it will be woken up to check for
ready tasks.

There are currently two tasks in the list. They have been extracted from the old service thread into the tasks:
`G1PeriodicGCTask` and `G1RemSetSamplingTask`. These are more or less identical to the old code, but in the form of
tasks. The only difference is that `G1PeriodicGCTask` no longer need to keep track of the last attempt, since it will
be rescheduled with the correct interval.

To reschedule a task is to update the time when it should be executed next and adding it back to the list. This
insertion is O(n) and it should be fine since the expected number of tasks handled by the service thread is low. If a
task don't want to be rescheduled it can set a negative timeout and that will cause the task to be dropped from the
list. There are currently no such tasks and adding such tasks come with the responsibility of making sure resources are
taken care of when the task is no longer active.

I've added a gtest to do some basic VM-testing and apart from a lot of local testing I've run mach5 tier 1-4 a few
times with good results.

I have created a few follow up tasks to look at moving the current tasks to more appropriate locations and also updated
a few old service thread related tasks with the label:
[`gc-g1-service-thread`](https://bugs.openjdk.java.net/issues/?jql=labels%20%3D%20gc-g1-service-thread)

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Commit messages:
 - Self review
 - 8254758: Change G1ServiceThread to be task based

Changes: https://git.openjdk.java.net/jdk/pull/734/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=734&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254758
  Stats: 544 lines in 3 files changed: 448 ins; 29 del; 67 mod
  Patch: https://git.openjdk.java.net/jdk/pull/734.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/734/head:pull/734

PR: https://git.openjdk.java.net/jdk/pull/734