Introduce Task quotas

In Gerrit 3.11, concept of TaskParker's is introduced to have the
ability to park a task without depriving other tasks of a thread. This
change uses that ability to introduce quotas on specific tasks.

Example:

[quota "*"]
  maxStartForTaskForQueue = 20 uploadpack SSH-Interactive-Worker
  maxStartForTaskForQueue = 10 uploadpack SSH-Batch-Worker

This gives more granular control of budgeting threads for specific kind
of tasks/queues.

Change-Id: Id12eee9ad44821dcbe4a74483ba688f7af500e9f
6 files changed