Use migrators for Storage

[Sergey Ukustov]

Currently Machinomy lib uses following DB engines:

• Postgresql
• Mongodb
• Nedb
• Sqlite

Migrations are ad hoc.

Proposal: Add explicit migrator to Storage subsystem, that handles database-related details on how to manage schema and data.

Expected API, per engine:

storage.migrate: Migrator

Migrator.isLatest: Promise<boolean> = returns true if latest migration is applied

Migrator.up(n?): Promise<void> = migrates up to n migration. Actual type of n is to be discovered based on underlying migration mechanism.

Migrator.down(n?): Promise<void> = migrates down to n migration. The same notice regarding n as for up is applied.

If the very concept of migrations do not make sense, provide dummy methods.

Start with Sqlite and Postgresql.

Use existing migration mechanism.

Expose migration strategy to Machinomy options as migrate option:

• silent silently applies migrations where needed, default value.
• raise raise an error if some migrations are pending.

Additionally, append mc_ prefix to all the tables managed by the library.

Additionally, expose SQL-schema-aware things from code logic to Migrator.

BTW, get rid of Mongodb.