Major dereferencing optimizations and fix for de-pickling outdated documents [migrated 921]

[reallistic]

Fixes #888:

• When de-pickling a document that is not dynamic, the _fields_ordered of the current type should be used
  • When de-pickling a document that is dynamic, the pickled _fields_ordered should be used for the current document ONLY

Dereferencing Optimizations:

• Compiles the set of dbrefs as a set instead of a list so that conversions don't need to be made later
• Checks for resolved objects via dict x in dict instead of list x in list.

Below is a preview of how the dereferencing optimizations improved loading 500 documents:

name	# of operations	name	# or operations
..bson/objectid.py:288 ObjectId.eq	250998	..bson/objectid.py:288 ObjectId.eq	998

(This output was generated by GreenletProfiler)

The left shows the master branches performance when retrieving 500 documents all of which have 2 referenced fields.
The right shows the same when using this pull request.
Please note that the left displays O(n^2) where as the right displays O(n). Although the average time for each operation is 0.000005 (not shown above) as n increases this time becomes incredibly large.
I found this issue when compiling 67,000 records took 1 hour

[landscape-bot]

Repository health decreased by 0.01% when pulling a6a7cba on YoApp:optimization_issue888 into 1ce6a7f on MongoEngine:master.

• 1 new problem was found (including 0 errors and 1 code smell).
• No problems were fixed.

[coveralls]

Coverage decreased (-0.02%) to 73.06% when pulling a6a7cba on YoApp:optimization_issue888 into 1ce6a7f on MongoEngine:master.

[coveralls]

Coverage decreased (-0.02%) to 73.06% when pulling a6a7cba on YoApp:optimization_issue888 into 1ce6a7f on MongoEngine:master.

[coveralls]

Coverage decreased (-0.02%) to 73.06% when pulling a6a7cba on YoApp:optimization_issue888 into 1ce6a7f on MongoEngine:master.

[coveralls]

Coverage decreased (-0.02%) to 73.06% when pulling a6a7cba on YoApp:optimization_issue888 into 1ce6a7f on MongoEngine:master.

[coveralls]

Coverage decreased (-0.02%) to 73.06% when pulling a6a7cba on YoApp:optimization_issue888 into 1ce6a7f on MongoEngine:master.