Stop zeroing huge memAllocBuf() buffers (#1592)

Stop zeroing huge memAllocBuf() buffers (#1592)

memAllocBuf() buffers smaller than 64KB were not zeroed before this
change. Larger buffers (a.k.a. "huge buffers") were zeroed with
xcalloc(). We believe it is safe to stop zeroing those huge buffers
because all known code that allocates huge buffers also allocates
smaller ones for the same purpose. If some of that code relied on
zeroing for years, we would expect to see problems with smaller buffers.

Removing xcalloc() allows removal of mem*Rigid(), memBufStats() and all
string pools API complications.

The cache manager mgr:mem report section for string
statistics is also removed.