Feature - ProgramCache index implementation switch

[Lichtso]

Problem

The current index structure in ProgramCache is to complex to reason about and maintain. A simpler index structure which uses the account-last-written-to-slot from the accounts-db instead of a slot version sequence is planned to replace it.
However, we want to be able to switch between the implementations easily to make sure they agree before removing the old one.

Summary of Changes

• Adds the enum IndexImplementation as a global switch.
• Inserts match self.index in all methods of ProgramCache which use entries or loading_entries.
• Removes direct usage of ProgramCache::entries in tests.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.73684% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 82.1%. Comparing base (e621336) to head (8e87677).
Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #1269    +/-   ##
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  Coverage    82.1%    82.1%            
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  Files         899      899            
  Lines      237263   237296    +33     
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+ Hits       194810   194913   +103     
+ Misses      42453    42383    -70