Set TotalEnvSteps as the default Tensorboard x-axis

[naeioi]

This PR sets TotalEnvSteps as the default tensorboard scalar log x-axis. This PR depends on rlworkgroup/dowel#38.

[ryanjulian]

We need to merge the dowel changes first, release a dowel version, then bump the version in garage.

[avnishn]

LGTM. Additionally, I like this feature, it is useful for comparing against other paper results that log in this fashion

[codecov]

Codecov Report

Merging #1069 into master will increase coverage by 4.14%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #1069      +/-   ##
==========================================
+ Coverage      82%   86.15%   +4.14%     
==========================================
  Files         161      161              
  Lines        7809     7807       -2     
  Branches      992      992              
==========================================
+ Hits         6404     6726     +322     
+ Misses       1208      890     -318     
+ Partials      197      191       -6

Impacted Files	Coverage Δ
src/garage/experiment/experiment_wrapper.py	86.04% <50%> (+0.55%)	⬆️
src/garage/misc/tensor_utils.py	88.23% <0%> (-2.36%)	⬇️
...rc/garage/sampler/off_policy_vectorized_sampler.py	97.75% <0%> (+1.12%)	⬆️
src/garage/tf/algos/npo.py	95.29% <0%> (+1.56%)	⬆️
src/garage/envs/base.py	91.37% <0%> (+1.72%)	⬆️
src/garage/tf/misc/tensor_utils.py	76.11% <0%> (+2.98%)	⬆️
.../exploration_strategies/epsilon_greedy_strategy.py	100% <0%> (+3.7%)	⬆️
src/garage/tf/policies/categorical_cnn_policy.py	100% <0%> (+4.25%)	⬆️
src/garage/tf/baselines/continuous_mlp_baseline.py	100% <0%> (+4.54%)	⬆️
src/garage/tf/algos/vpg.py	81.81% <0%> (+9.09%)	⬆️
... and 11 more

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[naeioi]

This PR will be ready when dowel 0.0.3 is tagged.

[ryanjulian]

Let me know when this is ready and I will force-push it.

[naeioi]

@ryanjulian If we don't care about the codedev, then this should be ready.
The uncovered code is the serializer in the experiment wrapper.