Refine P2PSync

[junshi15]

This PR makes the following changes into P2PSync class.

P2PSync::GetInitIter() ... a new getter function for initial iteration number.
P2PSync::run() ... refactored into 2 steps: prepare(), and thread launch/execution
P2PSync::prepare() ... establish the GPU pairs within a process for reduce operations

[shelhamer]

Please squash the lint fixes -- we like to keep the history clear.

[longjon]

This should probably be Prepare according to the style guide and existing style. Since we already have run, however, I guess we can postpone that to a future commit.

[longjon]

Thanks, this patch looks good except for the getter name issue above, an optionally fix up existing naming issues (member functions that do nontrivial work should be capitalized CamelCase.)

[junshi15]

Thanks for the review. We have revised the code accordingly.

[junshi15]

It looks there is a problem with travis-CI PYTHON-3.0 builds. It's not a problem with this PR. I will try to build again once that is fixed.

[longjon]

Okay, this looks good except for the (more subtle) point in the comment above; feel free to adjust accordingly and rebase on master which should fix the Travis issue.

[longjon]

Thanks for the patch, the diff and history look clean now, merging.