Node discovery logic.

Joins table together when a new node appears.

Things that make discovery logic harder:

- A table list is dynamic (but eventually we add all the tables into it).

- Creating Erlang distribution connection is async, but it net_kernel:ping/1 is blocking.

- net_kernel:ping/1 could block for unknown number of seconds (but net_kernel default timeout is 7 seconds).

- Resolving nodename could take a lot of time (5 seconds in tests). It is unpredictable blocking.

- join tables should be one by one to avoid OOM.

- Backend:get_nodes/1 could take a long time.

- cets_discovery:get_tables/1, cets_discovery:add_table/2 should be fast.

- The most important net_kernel flags for us to consider are:

* dist_auto_connect=never

* connect_all

* prevent_overlapping_partitions

These flags change the way the discovery logic behaves. Also the module would not try to connect to the hidden nodes.

Retry logic considerations:

- Backend:get_nodes/1 could return an error during startup, so we have to retry fast.

- There are two periods of operation for this module:

* startup phase, usually first 5 minutes.

* regular operation phase, after the startup phase.

- We don't need to check for the updated get_nodes too often in the regular operation phase.