Support Elasticsearch 7.0 and Guillotina 6

[masipcat]

Needs this plone/guillotina#930

PR #58 and #59 can be closed after merging this

[vangheem]

so everything seems okay but it's a lot to take in and know the context of every change since it's a mix of forward port + a few api changes.

The api changes seem fine and everything else looks fine.

Better yet, if we can finally stabilize tests, that'd be amazing!

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@df5d37c). Click here to learn what that means.
The diff coverage is 81.52%.

@@            Coverage Diff            @@
##             master      #66   +/-   ##
=========================================
  Coverage          ?   77.42%           
=========================================
  Files             ?       29           
  Lines             ?     2746           
  Branches          ?        0           
=========================================
  Hits              ?     2126           
  Misses            ?      620           
  Partials          ?        0

Impacted Files	Coverage Δ
guillotina_elasticsearch/parser.py	17.39% <ø> (ø)
guillotina_elasticsearch/tests/conftest.py	100% <ø> (ø)
guillotina_elasticsearch/queries.py	100% <ø> (ø)
guillotina_elasticsearch/tests/fixtures.py	92.68% <100%> (ø)
guillotina_elasticsearch/events.py	100% <100%> (ø)
guillotina_elasticsearch/tests/test_search.py	100% <100%> (ø)
guillotina_elasticsearch/reindex.py	71.87% <100%> (ø)
...uillotina_elasticsearch/tests/test_custom_index.py	100% <100%> (ø)
guillotina_elasticsearch/tests/test_parser.py	31.25% <100%> (ø)
guillotina_elasticsearch/tests/test_vacuum.py	100% <100%> (ø)
... and 13 more

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