Suboptimal debug code in mem_init_buf() and mem_erase_buf()

Callgrind simulated profiling of compact_compressed testcase on 5.6 debug configuration shows it spending ~56% of its cycles in ut_rnd_gen_ibool(), which is only called from UNIV_MEM_DEBUG-enabled mem_init_buf() / mem_erase_buf(), with a significant numbers of calls coming from UNIV_ZIP_DEBUG page_zip_validate().

Both mem_init_buf() and mem_erase_buf() fill the buffer with one of the two possible values, chosen randomly for each individual byte:

for each byte
       if (ut_rnd_gen_ibool()) {
                       *ptr = value1;
               } else {
                       *ptr = value2;
       }

This is very inefficient (reported upstream as http://bugs.mysql.com/bug.php?id=68725), and easy to replace with memset(), which would lose the randomness but would be much faster. Adding such memsets() to innodb56.patch shows compact_compressed testcase cycles to be cut 3x. There are similar results on 5.5 debug configuration as well.

Thus let's add this to XtraBackup InnoDB patches for improved testcase runtime.