Check DB connection when the application start

[tejash-jl]

Describe the bug
Currently, the application starts without the knowledge of the DB connection. If the downstream DB is down, we get to know only when we hit the API.

To Reproduce

1. Start the application with a wrong DB connection
2. Application starts
3. Call any API, fails with DB connection is not available.

Expected behavior
The application should not start if the DB connection is not possible.

Screenshots/Logs
dial tcp [::1]:5432: connect: connection refused

[sivachandran]

@tejash-jl are you working on this(asking as it is assigned to you)?

[tejash-jl]

@sivachandran currently I'm not

[sivachandran]

Okay, thanks @tejash-jl. Let me take a shot at it.

I'm thinking of introducing Ping function to repository.DBInterface and call pg.DB.Ping from postgres.Ping. Let me know if you can think of a better way.

@priyaaank