Change Footer version handling, Make compression dynamic
Change Footer version handling
Simplify version handling by switching to JSON instead of binary serialization.
fixes#1058
Make compression dynamic
Instead of choosing the compression during compile time via a feature flag, you can now have multiple compression algorithms enabled and decide during runtime which one to choose via IndexSettings. Changing the compression algorithm on an index is also supported. The information which algorithm was used in the doc store is stored in the DocStoreFooter. The default is the lz4 block format.
fixes#904
Handle merging of different compressors
Fix feature flag names
Add doc store test for all compressors