Updates to JointStore and MapBuilder

[shyamd]

• Updated JointStore to be able to merge across the lookup to the root level.
  This makes the aggregation look seamless to the person using the JointStore.
• Modified MapBuilder to get_keys and then chunk those when requesting data.
  This allows the MapBuilder to run in incremental mode by default.

Todo: Still thinking about how the index functions should be incorporated into MongoStore such that MapBuilder is not calling pymongo functions.

[coveralls]

Pull Request Test Coverage Report for Build 449

• 72 of 82 (87.8%) changed or added relevant lines in 7 files are covered.
• 19 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-1.07%) to 76.457%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
maggma/examples/builders.py	20	21	95.24%
maggma/examples/tests/test_copybuilder.py	3	4	75.0%
maggma/cli/mrun.py	0	2	0.0%
maggma/tests/test_advanced_stores.py	24	26	92.31%
maggma/advanced_stores.py	21	25	84.0%

Files with Coverage Reduction	New Missed Lines	%
maggma/advanced_stores.py	1	72.73%
maggma/tests/test_advanced_stores.py	1	95.85%
maggma/cli/mrun.py	1	0.0%
maggma/utils.py	16	52.34%

Totals
Change from base Build 424:	-1.07%
Covered Lines:	1614
Relevant Lines:	2111

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💛 - Coveralls

[shyamd]

Ok, This finally works. I think the test is a little mangled, but it was the only way I could get MongoDB 3.6 on the old trusty image that TravisCI seems to use.