Many headers don't really need to include database.hh, the include can
be replaced by forward declarations and/or including the actually needed
headers directly. Some headers don't need this include at all.
Each header was verified to be compilable on its own after the change,
by including it into an empty `.cc` file and compiling it. `.cc` files
that used to get `database.hh` through headers that no longer include it
were changed to include it themselves.
This method allows for querying a range or ranges on all shards of the
node. Under the hood it uses the multishard_combining_reader for
executing the query.
It supports paging and stateful queries (saving and reusing the readers
between pages). All this is transparent to the client, who only needs to
supply the same query::read_command::query_uuid through the pages of the
query (and supply correct start positions on each page, that match the
stop position of the last page).
