With a large number of table the schema mutations
vector might get big enoug to cause reactor stalls
when freed.
For example, the following stall was hit on
2023.1.0~rc1-20230208.fe3cc281ec73 with 5000 tables:
```
(inlined by) ~vector at /usr/bin/../lib/gcc/x86_64-redhat-linux/12/../../../../include/c++/12/bits/stl_vector.h:730
(inlined by) db::schema_tables::calculate_schema_digest(seastar::sharded<service::storage_proxy>&, enum_set<super_enum<db::schema_feature, (db::schema_feature)0, (db::schema_feature)1, (db::schema_feature)2, (db::schema_feature)3, (db::schema_feature)4, (db::schema_feature)5, (db::schema_feature)6, (db::schema_feature)7> >, seastar::noncopyable_function<bool (std::basic_string_view<char, std::char_traits<char> >)>) at ./db/schema_tables.cc:799
```
This change returns a mutations generator from
the `map` lambda coroutine so we can process them
one at a time, destroy the mutations one at a time,
and by that, reducing memory footprint and preventing
reactor stalls.
Fixes#18173
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
95a5fba0ea