Add mutation_source_metadata
Add mutation_source_metadata

This struct contains metadata regarding to a mutation_source. Currently
it contains the min and max timestamp. This will be used later by
compaction strategies to determine whether a given mutation stream has
to be split or not.

scylladb/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla