Adding a new tenant needs to be done under cluster feature protection.
However it wasn't the case for adding `$maintenance` statement tenant
and to fix it we need to support an upgrade from node which doesn't
know about maintenance tenant at all and from one which uses it without
any cluster feature protection.
This commit adds `enabled` flag to statement tenants.
This way, when the tenant is disabled, it cannot be used to create
a connection, but it can be used to accept an incoming connection.
d44844241d