Fixed a memory leak issue in InternalThread (and removed caffe::Thread)

[ryotat]

This PR fixes the memory leak issue I posted here:
#1316 (comment)

[shelhamer]

caffe::Thread is required for cross-platform compatibility -- see #1009 and #1010. Restore the wrapper but keep your fix.

[seanbell]

I think that there should be a comment above the wrapper explaining its purpose. Otherwise it seems useless. I deleted it in my fork, not realizing that it had a use on other platforms.

[shelhamer]

Good idea. @ryotat please add a comment to explain the caffe::Thread facade like the one for boost RNG here: https://github.com/BVLC/caffe/blob/master/include/caffe/common.hpp#L83-L84

[ryotat]

How about forward declaring boost::thread instead of using a wrapper class? Could someone try my commit 85477b7 if it compiles correctly on OSX?

[shelhamer]

Thanks @ryotat -- forward declaration worked fine on OS X. I linted the code after merge, but please remember to do make lint for PRs.