refs #5710 add FT_MSG_KEYVAL_BYTES_IN to measure the amount of key/value userdata ingested at the root. this value does not count message/xid overhead so it is a good denominator when calculating write amplification


git-svn-id: file:///svn/toku/tokudb@50404 c7de825b-a66e-492c-adef-691d508d4ae1