Fix bugs in xmalloc, xrealloc. Add docs.
Fix bugs in xmalloc, xrealloc. Add docs.

DavidLi2010/ramcloud,taschik/ramcloud,jcarreira/ramcloud,taschik/ramcloud,behnamm/cs244b_project,DavidLi2010/ramcloud,rstutsman/RAMCloud,utah-scs/RAMCloud,IMCG/RamCloud,taschik/ramcloud-load-manager,Frank-Wu/RamCloud,jcarreira/ramcloud,Frank-Wu/RamCloud,Frank-Wu/RamCloud,utah-scs/RAMCloud,behnamm/cs244b_project,y-higuchi/ramcloud,alexandermerritt/ramcloud,SMatsushi/RAMCloud,mrdiegoa/ramcloud,taschik/ramcloud,IMCG/RamCloud,alexandermerritt/ramcloud,QingkaiLu/RAMCloud,behnamm/cs244b_project,mrdiegoa/ramcloud,y-higuchi/ramcloud,IMCG/RamCloud,taschik/ramcloud-load-manager,DavidLi2010/ramcloud,anirajk/RAMCloud,SMatsushi/RAMCloud,taschik/ramcloud,matrix207/RAMCloud,jblomer/ramcloud,Frank-Wu/RamCloud,Frank-Wu/RamCloud,rstutsman/RAMCloud,rstutsman/RAMCloud,IMCG/RamCloud,anirajk/RAMCloud,QingkaiLu/RAMCloud,rstutsman/RAMCloud,alexandermerritt/ramcloud,DavidLi2010/ramcloud,jcarreira/ramcloud,alexandermerritt/ramcloud,DavidLi2010/ramcloud,anirajk/RAMCloud,mrdiegoa/ramcloud,jblomer/ramcloud,jblomer/ramcloud,jcarreira/ramcloud,taschik/ramcloud-load-manager,DavidLi2010/ramcloud,taschik/ramcloud-load-manager,y-higuchi/ramcloud,rstutsman/RAMCloud,jcarreira/ramcloud,matrix207/RAMCloud,behnamm/cs244b_project,QingkaiLu/RAMCloud,alexandermerritt/ramcloud,taschik/ramcloud,behnamm/cs244b_project,mrdiegoa/ramcloud,y-higuchi/ramcloud,mrdiegoa/ramcloud,matrix207/RAMCloud,Frank-Wu/RamCloud,utah-scs/RAMCloud,jblomer/ramcloud,anirajk/RAMCloud,utah-scs/RAMCloud,rstutsman/RAMCloud,SMatsushi/RAMCloud,matrix207/RAMCloud,jblomer/ramcloud,SMatsushi/RAMCloud,taschik/ramcloud-load-manager,SMatsushi/RAMCloud,alexandermerritt/ramcloud,QingkaiLu/RAMCloud,matrix207/RAMCloud,behnamm/cs244b_project,matrix207/RAMCloud,QingkaiLu/RAMCloud,y-higuchi/ramcloud,utah-scs/RAMCloud,y-higuchi/ramcloud,QingkaiLu/RAMCloud,utah-scs/RAMCloud,mrdiegoa/ramcloud,jblomer/ramcloud,anirajk/RAMCloud,IMCG/RamCloud,IMCG/RamCloud,anirajk/RAMCloud,SMatsushi/RAMCloud,jcarreira/ramcloud