No last time index update on normalize

[rwedge]

This PR makes some changes to when last time indexes are recalculated.

• recalculate_last_time_indexes will only recalculate if the entity had a last time index
• Not recalculating last time index values after normalizing an entity. This can really slow down EntiySet creation. I think using EntitySet.add_last_time_indexes after getting the final EntitySet structure is simple enough.
• During EntitySet.concat, last time indexes are updated in one go instead of when updating each entity's data.

[codecov-io]

Codecov Report

Merging #169 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #169      +/-   ##
==========================================
+ Coverage   92.88%   92.93%   +0.05%     
==========================================
  Files          70       70              
  Lines        7486     7519      +33     
==========================================
+ Hits         6953     6988      +35     
+ Misses        533      531       -2

Impacted Files	Coverage Δ
featuretools/entityset/entityset.py	93.78% <100%> (+0.22%)	⬆️
featuretools/entityset/entity.py	85.57% <100%> (+0.24%)	⬆️
featuretools/tests/entityset_tests/test_es.py	99.76% <100%> (+0.01%)	⬆️

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[kmax12]

Looks good, merging