[t:3635] [t:3764] [t:3757] [t:3749] merging tokudb.3635+prefetch into mainline, pending testing, fixes #3635, #3764, #3757, #3749

git-svn-id: file:///svn/toku/tokudb@33537 c7de825b-a66e-492c-adef-691d508d4ae1