In Classic modes (CS\SC) every attachment have its own private cache of allocation table contents.
Access to allocation table is protected by common allocation table lock (alloc lock).
Currently, when attachment needs to look for a page number at allocation table, it acquires alloc lock in SH mode.
But it is not necessary if corresponding part of allocation table is already cached by attachement as allocation
table entries is never changed, it is only appended.
More, a lot of unnecessary requests for a SH lock not allows writers to proceed as quick as it can.
The improvement is to introduce additional local (per-attachment) read-write lock to protect access to the
attachment's private cache of the allocation table. It allows to significantly lower contention on global lock.