Based on SERVER-108426 add a new ticket pool that will serve tickets to operations marked as low-priority. The scope of this ticket includes:

• Add the new pool.
• Add the server parameters to enable or disable the pool and control the number of tickets of each pool.
• Add the respective unit tests and necessary jstests to check for basic correctness (more in depth testing will be added later on).
• Add the RAI type to mark an operation as low priority.
• Mark the TTL indexes as a low priority operation (this will be used by follow up tickets to mark other operations as low priority).