This commit attempts to address the feedback in
https://github.com/rabbitmq/rabbitmq-server/pull/5254#discussion_r924497137.
Delete special code paths like function rabbit_table:ensure_feature_flag_tables/0
as they are confusing.
Instead, handle the new rabbit_index_route table as any other Mnesia
table if feature flag direct_exchange_routing_v2 is enabled.
Note that this commit comes with one important restriction:
A feature flag migration function MUST NOT call
rabbit_table:definitions/0 (directly or indirectly) as this will result
in a deadlock where the migration function is stuck on the global lock
with resource ID 'feature_flags_state_change' when checking for
rabbit_feature_flags:is_enabled(direct_exchange_routing_v2)
because the migration function runs in a different (RPC) process than
the function that holds the 'feature_flags_state_change' lock.
The latter lock is set before a feature gets enabled and released
after the feature flag got enabled.