added context manager for stores

[jmmshn]

• added context manager for stores
• set index key with kwarg in S3Store.init
• changed test case so we actually test S3Store.update(dict) and S3Store.update(List[dict])

[codecov]

Codecov Report

Merging #258 into master will decrease coverage by 0.04%.
The diff coverage is 70.00%.

@@            Coverage Diff             @@
##           master     #258      +/-   ##
==========================================
- Coverage   87.64%   87.59%   -0.05%     
==========================================
  Files          27       27              
  Lines        2007     2016       +9     
==========================================
+ Hits         1759     1766       +7     
- Misses        248      250       +2     

Impacted Files	Coverage Δ
src/maggma/core/store.py	85.56% <40.00%> (-2.48%)	⬇️
src/maggma/stores/aws.py	81.52% <100.00%> (+1.13%)	⬆️

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