Move root_net_ check in net constructor

[junshi15]

This PR extends recurrent_layer to the multi-gpu settings.

Currently, the recurrent_layer constructs a net internally (the unrolled net), https://github.com/BVLC/caffe/blob/master/src/caffe/layers/recurrent_layer.cpp#L108.
No root_net_ is set in that constructor. In a multi-gpu setting, worker solver will create an unrolled net too, however it will fail at the line below, https://github.com/BVLC/caffe/blob/master/src/caffe/net.cpp#L50-L51, because the worker solver is not a root solver and the net does not have a root_net.

Since root_net is only used in share_from_root, this PR moves the check to where share_from_root is defined.

[shelhamer]

Heads-up: the root_net_ is going away entirely as part of the simplifications in #4563.

[junshi15]

That's a large PR, may take a while. It may make sense merging this one first.

Similar complaints here: #4851

[shelhamer]

This should be unnecessary with the merge of #4563, but thank you for proposing a fix. @junshi15 and @SIshijima could you check that #4563 in fact fixes the issue?

[junshi15]

@shelhamer I won't have time to verify the recurrent layer at this moment as I am quite busy at work. Let me close this PR for now. If we see problems in the future, we will re-visit it.