Overflow buffer allocation for CCTL threads

OBA for CCTL threads is similar to that for BMPs. The OBA value used for each thread is set with the FPOB parameter in the startup table.

This buffer allocation is optional and is used for exceptional buffer requirements when the FPB has been exhausted. Its use is dependent on obtaining a latch that serializes all BMPs and CCTL threads currently in an overflow buffer state. If the latch is not obtained, the FPB value is increased by the FPOB value, and normal processing resumes. The overflow buffer latch is released during sync point processing. At any point in time, only the largest OBA/FPOB request among all the active BMPs and CCTL threads is page fixed in the fast path buffer pool.