cache external postgres port in Branch model

[boddumanohar]

fixes: simplyblock/vela#181

Currently as a part of _public of branch the node_port of a service is retrieved from API server. But this port number never changes. So this can be safely stored in branch model.

testing

after migration upon calling the /branch endpoint, the DB got populated

postgres=# select db_port from branch;
 db_port 
---------
        
        
(2 rows)

postgres=# select db_port from branch;
 db_port 
---------
   31432
   31444
(2 rows)

postgres=# 

[mxsrc]

I think a cleaner approach would be a single helper to access the port, returning the branch attribute if present, if not, looking up and persisting the port. Ideally, it'd fail when the lookup fails as well.

This would avoid the fallbacks on access, avoid having to call the _persist function in multiple places, which is a code smell, and avoid having to establish the values during the migration.

[boddumanohar]

thanks, that's actually a great idea. I've implemented the changes and update the PR description with the testing results.