What kind of change does this PR introduce?

Before Realtime Channel join it checks if we're able to connect to Tenant database. If not we exit and avoid the client from connecting with the Channel.

Current procedure:

• One running GenServer for each node in the node closest to the tenant
• On GenServer init/1 check database connection with Postgrex and a SELECT 1 query
  • If successful, store in GenServer metadata the connection
  • If failed, GenServer dies
• GenServer tracks shutdown of DBConnection process and dies, cleaning the :syn registry in the process

Notes:

• Includes some refactors to
  • SynHandler made more reusable by allowing multiple modules
  • Realtime.Nodes made with common functions required for node management
  • Realtime.Helpers with extra functions for future refactors

Future Work

Later we will want to evolve this feature to a more solid health check of the tenant database connection.