Search:search_multiclass & learner_id

@hal3 

It seems that learner_id is not setted correctly in search.cc. Also, there are a few bugs in search_multiclass.cc. Please let me know if you have any comment about my modifications.

Hal: could you check line 1028 and line 1058 in search.cc? I'm not sure I should pass "policy" or "learner" to cached_action_store_or_find function. Thanks.

The performance of search_er is improved because of the bug fix of learner_id.

I also add a test case for search_multiclass.

Thanks,