Rename Frame._vecs ==> Frame.vecs()

Also make closing Appendables more lazy.
Remove some unneeded AppendableVec uses - normal Vecs are easier/faster.
Rename ._vecs[].length ==> .numCols()
Rename ._vecs[0].length() ==> .numRows()
Merge branch 'master' of github.com:0xdata/h2o
Remove debug print, unwind overlapp work
Proper blocking after closeAppendables
Pull Frame Vecs from the DKV instead passing them
Merge w/master
Merge branch 'cliffc-gbm6'
Merge branch 'master' of github.com:0xdata/h2o