Refactor prefetch
test=release/1.2

test=develop

Fix code to support cpplint syntax check

test=develop

* wip

* add ref_by_trainer_id op

* ready to test

* fix ref inputs

* refine rpc_op_handle

* fix merge bug

test=develop

test=develop

test=develop

Add rpc profiler flags

Add grpc error context

Add rpc timeline

test=develop

test=develop

test=develop

test=develop

* wip

* clean up

* should fix running with memopt

* add ut

* mark lr schedule op role

* hide lr_schedule_guard

* use op_role_var instead of ufind

* unify dist test name

* wip for py3 support

* fix var deref

* fix python3 mem_opt order

* remove comments

Optimize selected rows for dist lookup table with rwlock 

Add a few more RecordEvent.
Cleanup

* refine regularization for selected_rows

* clean lookup_table

* refine rpc_server_test

* temporally disable rpc_server_test

* fix rpc_server_test

* add unit test