Refactor ScanStage stage to improve performance.

Suggested changes:

• Split ScanStage into SeekStage, ScanStage and RandomScanStage. Those operations are mutually exclusive. Splitting the stages will reduce the method sizes and reduce the amount branching on the hotpath.
• Remove unnecessary virtual calls from IndexScan stage. If code sharing between Scan stage variants is desired a templates can be used for methods that would need to otherwise call virtual methods.
• Refactor code and branching so that open and getNext have minimal size. All expensive/optional operatations can be extracted to separate methods to discourage inlining of code that is often skipped.
• Encourage inlining of small methods on the hot path.
• Reduce frequency of calling into YieldPolicy, at minimum we should be able to gate the yield checks behind the counter even if yield policy is present.

 A POC here https://github.com/10gen/mongo/compare/anna.wawrzyniak/SERVER-85623?expand=1 demonstrates the above changes and possible performance improvement (see SERVER-85623)