[Persistence] Implement proper SQL migration strategy (i.e. move SQL table schemas to .sql files)

[andrewnguyen22]

@Olshansk
For this file (persistence/schema/account.go) and all the others, I wanted to add a point about migrations & schemas that I feel pretty strongly about before moving forward.

I looked at different SQL migration tools in Golang and though I haven't picked a specific one, golang-migrate/migrate seems to be the most popular one. However, instead of defining our tables in strings withing .go file, we should follow the best practices for how to manage migrations (note that creation is a migration in itself).

Source: golang-migrate/migrate
tl;dr

We store these in .sql files
Each change will have a .up.sql and a .down.sql file
This will enable:
Cleaner & easier to read code (separation of go and SQL)
Going back & forth during development
Easy to track changes / migrations when we actually do them on chain.

[derrandz]

Adding to this a comment I put as a review for #73

We can make use of the following tool: dbmate. This tool will basically run in a container alongside the database, and will receive the database URL as an environment variable.

This will allow us to create very simple migrations in SQL, by simply using text directives:

-- migrate:up

-- migrate:down

So an example of a migration would be:

-- migrate:up
create table users (
  id integer,
  name varchar(255),
  email varchar(255) not null
);

-- migrate:down

This migration file will be generated using a command (dbmate new create_users_table) and we will fill it however we see fit.

Then, running the migrations up or down becomes a matter of calling a make command that will send the instruction to the tool in the container, which will subsequently run it against the database, the command would be something like:

migration-up:
	docker exec -v $(PWD)/migrations:/db migrater dbmate up

migration-down:
	docker exec -v $(PWD)/migrations:/db migrater dbmate rollback

And to generate new timestamped migrations:

generate_migration:
   docker exec -v $(PWD)/migrations:/db migrater dbmate new action_table