This means that Caffe::Get has to be moved to common.cpp, and loses its
"inline" (but there are no real performance implications).

Instead of just keeping track of input and output blobs, also keep track
of layer dependencies. (Also adjust AppendBottom's argument types to
avoid passing an input as a pointer.)

This simplifies the OS X build, and will allow use of the per-thread
default stream for running existing layer code asynchronously.

Note that this may cause issues with code that assumes either explicit
or device-level synchronization, which we'll fix in the next commit.

This ensures that layers are synchronous with respect to each other,
even when layer code doesn't use explicit streams.

There are no cases where Forward is called without Reshape, so we can
simplify the call structure.

This will allow us to cleanly kill compute threads that are waiting for
wark.

This gives us a way to specify layer-level execution placement for
layerwise parallelism, implemented in future commits.

Split layer gains a param, top_device, which allows tops to exist on
different (explicitly specified) devices. Params are automatically
copied and diffs are automatically accumulated. Because the
implementation is now device-agnostic, it's done in (only) the *_cpu
functions.

This fills in the top_device param of split layer according to the
device params of the connecting layers.

This is necessary to ensure that buffers are allocated on the correct
devices.

Compute threads hold (blocking) queues of forward or backward commands,
which are synchronized according to the layer graph through Net member
variables.

This fully exercises the multi-GPU case, and saves time.

This is necessary to ensure that operations are performed on the correct
device.