Replace init task with seed values

[sangaman]

Closes #331.

This commit makes xud add seed values for pairs and currencies when a new database is created during startup. These are in addition to the seed nodes. An initDb config option can be set to false to disable initializing new databases with these seed values. This makes the db:init task obsolete.

I didn't add a value to the [db] config section because we've had that line up with Sequelize options and initValues would have broken that. I tried this approach at first but it was getting messy making changes to DB.ts to accommodate it. So instead I put initDb as a top level config option.

[moshababo]

Looks good, thanks.

Please remove the init task from the tasks index file and the gulpfile as well. We might add it again later, to be used for test automation (with different values), but for now it's not needed.

[kilrau]

works like a charme, but it's still based on hosts, so you might have adjust this to nodes as per #315 ? @sangaman

[sangaman]

Yup, I will refactor once #315 is merged.

Edit: Which should be now, one moment.

[sangaman]

@kilrau Rebase done, do you want to try this again one more time now that it's using nodes?

[kilrau]

like a charme!