Bypass the expensive check to the allocator getStats() when obtaining inuse data

Bypass the expensive check to the allocator getStats() when obtaining inuse data

A lot of busy code calls getStats() to get the inuse values when calculating
queue length (diskthreads) and store memory use for object memory expiry.
This was destroying performance.

The changes implement a "shortcut" to obtain the inuse values for both the
malloc and mempool allocator. getStats() is still expensive for mempools
(as it does GC before updating all the statistics to get accurate information)
but code has been modified to not use getStats() where all it wanted was
inuse info.