MLP: Fix step size initialization based on learning rate policy
MLP: Fix step size initialization based on learning rate policy

JIRA: MADLIB-1212

The step_size is supposed to be updated based on the learning rate. The
formulae for different policies depend on the current iteration number,
which was not consumed correctly. This commit fixes it by using
it.iteration for the current iteration number in that update.

Co-authored-by: Nikhil Kak <nkak@pivotal.io>

Closes #240

iyerr3/incubator-madlib,hpandeycodeit/incubator-madlib,njayaram2/madlib,orhankislal/incubator-madlib,apache/incubator-madlib,apache/incubator-madlib,njayaram2/incubator-madlib,orhankislal/madlib,orhankislal/incubator-madlib,apache/incubator-madlib,orhankislal/madlib,njayaram2/incubator-madlib,orhankislal/madlib,hpandeycodeit/incubator-madlib,njayaram2/madlib,apache/incubator-madlib,madlib/madlib,orhankislal/madlib,madlib/madlib,orhankislal/incubator-madlib,njayaram2/madlib,hpandeycodeit/incubator-madlib,njayaram2/incubator-madlib,orhankislal/madlib,iyerr3/incubator-madlib,orhankislal/incubator-madlib,madlib/madlib,iyerr3/incubator-madlib,madlib/madlib,iyerr3/incubator-madlib,hpandeycodeit/incubator-madlib,hpandeycodeit/incubator-madlib,njayaram2/madlib,njayaram2/incubator-madlib,apache/incubator-madlib,orhankislal/incubator-madlib,madlib/madlib,njayaram2/incubator-madlib,iyerr3/incubator-madlib,njayaram2/madlib