define token_metadata_ptr in token_metadata_fwd.hh
So that the declaration of `make_splitter` can be moved
to token_range_splitter.hh, where it belongs,
and so token_metadata.hh won't have to include it.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Currently, scans are splitting partition ranges around tokens. This
will have to change with tablets, where we should split at tablet
boundaries.
This patch introduces token_range_splitter which abstracts this
task. It is provided by effective_replication_map implementation.
