tombstone_gc.hh is relatively lightweight and is used in many places,
but it includes the heavyweight boost/icl/interval_map.hh. Lighten
the load for its users by wrapping lw_shared_ptr<some icl map type>
in a forward-declared class. Define the class in a new header
tombstone_gc-internals.hh, to be used by the two translation units
that need it.
Ref #1.
Closesscylladb/scylladb#21706