Fix#605
All MergeTree tables are now replicated.
For some tables, a `_local` variant is added and the non-`_local`
variant is now distributed. The distributed tables are the `flows`
table, the `flows_DDDD` tables (where `DDDD` is a duration), as well as
the `flows_raw_errors` table. The `exporters` table is not distributed
and stays local.
The data is following this schema:
- data is coming from `flows_HHHH_raw` table, using the Kafka engine
- the `flows_HHHH_raw_consumer` reads data from `flows_HHHH_raw` (local)
and sends it to `flows` (distributed) when there is no error
- the `flows_raw_errors_consumer` reads data from
`flows_HHHH_raw` (local) and sends it to
`flows_raw_errors` (distributed)
- the `flows_DDDD_consumer` reads fata from `flows_local` (local) and
sends it to `flow_DDDD_local` (local)
- the `exporters_consumer` reads data from `flows` (distributed) and
sends it to `exporters` (local)
The reason for `flows_HHHH_raw_consumer` to send data to the distributed
`flows` table, and not the local one is to ensure flows are
balanced (for example, if there is not enough Kafka partitions). But
sending it to `flows_local` would have been possible.
On the other hand, it is important for `flows_DDDD_consumer` to read
from local to avoid duplication. It could have sent to distributed, but
the data is now balanced correctly and we just send it to local instead
for better performance.
The `exporters_consumer` is allowed to read from the distributed `flows`
table because it writes the result to the local `exporters` table.
