Intermediate language added new layer of abstraction between cql
statement and quering mutations, thus this commit adds new layer of
abstraction between mutations and returning query result.
Result can't be directly returned from `group0_state_machine::apply`, so
we decided to hold query results in map inside `raft_group0_client`. It can
be safely read after `add_entry_unguarded`, because this method waits
for applying raft command. After translating result to `result_message`
or in case of exception, map entry is erased.
Extended `group0_command` to enable transmission of `raft::broadcast_tables::query`.
Added `add_entry_unguarded` method in `raft_group0_client` for dispatching raft
commands without `group0_guard`.
Queries on group0_kv_store are executed in `group_0_state_machine::apply`,
but for now don't return results. They don't use previous state id, so they will
block concurrent schema changes, but these changes won't block queries.
In this version snapshots are ignored.
We decided to extend `cql_statement` hierarchy with `strongly_consistent_modification_statement`
and `strongly_consistent_select_statement`. Statements operating on
system.broadcast_kv_store will be compiled to these new subclasses if
BROADCAST_TABLES flag is enabled.
If the query is executed on a shard other than 0 it's bounced to that shard.
