Don't clear allocated memory if not required

28.84% pread64 (This seem like the unavoidable "real work")

21.49% __wt_cksum_hw (WT-2121 should help this)

46.00% memset from __wt_realloc from __wt_buf_grow_worker from __wt_block_read_off from __wt_bm_read from __wt_bt_read

The 46% is split into 37.38% servicing minor pagefaults (which would move somewhere else if we didn't memset) and ~8% in memset itself (which wouldn't).

The call to memset is prefaced with the following comment:

    /*

     * Clear the allocated memory -- an application might: allocate memory,

     * write secret stuff into it, free the memory, then we re-allocate the

     * memory and use it for a file page or log record, and then write it to

     * disk.  That would result in the secret stuff being protected by the

     * WiredTiger permission mechanisms, potentially inappropriate for the

     * secret stuff.

     */