cxgbe(4): Support routines for Tx traffic scheduling.

• Create a new file, t4_sched.c, and move all of the code related to traffic management from t4_main.c and t4_sge.c to this file.
• Track both Channel Rate Limiter (ch_rl) and Class Rate Limiter (cl_rl) parameters in the PF driver.
• Initialize all the cl_rl limiters with somewhat arbitrary default rates and provide routines to update them on the fly.
• Provide routines to reserve and release traffic classes.

MFC after: 1 month
Sponsored by: Chelsio Communications