Allow disabling tombstone gc on a per-query basis for mutation queries.
This is achieved by a bool flag passed to mutation query variants like
`query_mutations_on_all_shards()` and `database::mutation_query()`,
which is then propagated down to compaction_mutation_state.
The future user (in the next patch) is the SELECT * FROM
MUTATION_FRAGMENTS() statement which wants to see dead partitions
(and rows) when scanning a table. Currently, due to garbage collections,
said statement can miss partitions which only contain
garbage-collectible tombstones.
It belongs there, it is a completely replica-side thing. Also take the
opportunity to rename it to multishard_query.{hh,cc}, it is not just
mutation anymore (data query is also implemented).
