SearchResult API clean up

[scottinet]

• DRY-ify the construction of SearchResult instances: it's now up to this class to build itself from Kuzzle API results
• remove the need of providing a previous SearchResult instance to the SearchResult constructor: the fetchNext method is now in charge of updating the next SearchResult instance with the relevant information (namely, how many documents have been fetched so far)
• rename the fetchedDocument property into fetched
• fix the creation of the search_after filter configuration, derivated from the sort filter property, which may contain either an object or a string (only the object kind of property was handled)

Boyscout: covnerted SearchResult implementation to ES6

[codecov-io]

Codecov Report

Merging #267 into 6.x will decrease coverage by 84.71%.
The diff coverage is 64.44%.

@@             Coverage Diff             @@
##              6.x     #267       +/-   ##
===========================================
- Coverage   98.46%   13.74%   -84.72%     
===========================================
  Files          17       17               
  Lines        2079     2045       -34     
  Branches      604      595        -9     
===========================================
- Hits         2047      281     -1766     
- Misses         32     1764     +1732

Impacted Files	Coverage Δ
src/Collection.js	11.85% <0%> (-88.15%)	⬇️
src/SearchResult.js	68.18% <69.04%> (-26.82%)	⬇️
src/Room.js	2.06% <0%> (-97.94%)	⬇️
src/networkWrapper/protocols/socketio.js	3.44% <0%> (-91.38%)	⬇️
src/Kuzzle.js	7.97% <0%> (-89.88%)	⬇️
src/security/SecurityDocument.js	12.24% <0%> (-87.76%)	⬇️
src/security/Security.js	11.14% <0%> (-86.58%)	⬇️
src/networkWrapper/protocols/abstract/realtime.js	12.92% <0%> (-85.72%)	⬇️
src/CollectionMapping.js	12.19% <0%> (-85.37%)	⬇️
src/networkWrapper/protocols/websocket.js	15.68% <0%> (-84.32%)	⬇️
... and 8 more

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