Refactor IndexScan stage to improve performance.

Suggested changes:

• Remove unnecessary virtual calls from IndexScan stage. If code sahring between Simple and Generic index scan 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.
• Move loop invariants out of the loop. e.g. Some slot assignment is invariant and can be done in open/prepare phase, and no longer needs assignment in getNext.
• 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)