Added serveral tests in Perf.cc to illuminate the cost of   (a) Cold memcpy's   (b) Iterating a Buffer with multiple chunks   (c) Buffer appendCopy vs. appendExternal
Added serveral tests in Perf.cc to illuminate the cost of
  (a) Cold memcpy's
  (b) Iterating a Buffer with multiple chunks
  (c) Buffer appendCopy vs. appendExternal

rstutsman/RAMCloud,rstutsman/RAMCloud,SMatsushi/RAMCloud,alexandermerritt/ramcloud,rstutsman/RAMCloud,anirajk/RAMCloud,SMatsushi/RAMCloud,alexandermerritt/ramcloud,jblomer/ramcloud,rstutsman/RAMCloud,matrix207/RAMCloud,SMatsushi/RAMCloud,QingkaiLu/RAMCloud,matrix207/RAMCloud,QingkaiLu/RAMCloud,QingkaiLu/RAMCloud,jblomer/ramcloud,QingkaiLu/RAMCloud,utah-scs/RAMCloud,anirajk/RAMCloud,alexandermerritt/ramcloud,anirajk/RAMCloud,rstutsman/RAMCloud,utah-scs/RAMCloud,alexandermerritt/ramcloud,SMatsushi/RAMCloud,SMatsushi/RAMCloud,utah-scs/RAMCloud,jblomer/ramcloud,anirajk/RAMCloud,rstutsman/RAMCloud,anirajk/RAMCloud,QingkaiLu/RAMCloud,QingkaiLu/RAMCloud,utah-scs/RAMCloud,matrix207/RAMCloud,alexandermerritt/ramcloud,anirajk/RAMCloud,SMatsushi/RAMCloud,matrix207/RAMCloud,alexandermerritt/ramcloud,utah-scs/RAMCloud,utah-scs/RAMCloud,matrix207/RAMCloud,jblomer/ramcloud,matrix207/RAMCloud,jblomer/ramcloud,jblomer/ramcloud