datastore: .ef v2 format

[battlmonstr]

• added new snapshots encoder method: decode_word_with_metadata(); it is needed so that the new codec has info about the base timestamp of the file before decoding
• added a new type InvertedIndexTimestampList to represent II values, and refactored II queries to work with it instead of EliasFanoList32 directly
• InvertedIndexTimestampListDecoder implements the new encoding format
• refactored EliasFanoList32 data_holder_ to be the primary source of truth; this simplifies InvertedIndexTimestampListDecoder code, from_encoded_data overloads code and it is a step towards fixing the dangling span after move problem
• added step_size/make_step_converter to the repo schema so that bundles can be created with a proper step_converter
• elias_fano_decoder_test.cpp is merged into elias_fano_list_test.cpp