MemPools: remove zero on allocate (calloc) from some pool types

MemPools: remove zero on allocate (calloc) from some pool types

As we are closing defects identified by Coverity and improving
constructors everywhere we are creating a minor anti-pattern in
MemPool'ed objects with calloc() in the pool initializing the memory
then constructors re-initializing it in a better way for that object.

MemPools contains a doZeroOnPush flag to optimize performance by removing
use of memset() as chunks are added back into the pool. However, on
closer inspection it is clear that the following pop() process to re-use
those chunks is never performing memset() anyway. As such I believe that
there is no special need to use calloc() on these particular object types
in the first place.

Update MemPools to use malloc() instead of calloc() on all types with
doZeroOnPush set. This should increase performance a little, and allows
us to remove the anti-pattern by setting doZeroOnPush for more objects
as we can verify they are correctly initialized by their constructors.