Reduce `deepcopy` for `VarInfo`

[yebai]

The current implementation of HMC/NUTS contains many calls to deepcopy for VarInfo. This substantially increases memory allocation and may deteriorate performance. Perhaps we can share the same VarInfo between leapfrog iterations that belong to a single HMC/NUTS proposal (or sample).

[yebai]

TODOs

• Create separate builds for LDA and MoC, so that we can record performance changes for every commit.
• Measure and record rum time required for each leapfrog step.

[xukai92]

What does "create separate builds" mean?

[yebai]

At the moment, we have benchmark.jl and runtests.jl on Travis. Perhaps we should separate lda.run.jl and moc.run.jl into separate builds so that we don't exceed the build time limits imposed by Travis.

[xukai92]

I see!

[xukai92]

Resolved in #254