Programmable API not working with MongoDB

Currently db-migrate programmable API doesn't work properly with scope.
As there are two migration scopes (root and test) and so we intent to run `down on 'test' scope`, `down on root scope`, `up on root scope`, then `up on test scope` in sequence, however, the `db-migrate API` doesn't work properly as our intention. Once we run `down on test scope`, the coming up/down would run on the same scope.

To execute up and down in sequence with different scopes, `DBMigrateHelper` was added as a mediator and now we can mix up and down at our convenience (but we should invoke `resetInstance` method between up and down):
```
let dbm = DBMigrateHelper.getInstance();
DBMigrateHelper.executeDown(dbm, 'test')
.then(() => DBMigrateHelper.executeDown(dbm))
.then(() => DBMigrateHelper.resetInstanceAsync(dbm))
.then(() => DBMigrateHelper.executeUp(dbm))
.then(() => DBMigrateHelper.executeUp(dbm, 'test'))
.then(() => resolve());
```

`DBMigrateHelper`:

```
class DBMigrateHelper {
public static getInstance() {
return DBMigrate.getInstance(true);
}

public static resetInstance(dbm: any): void {
if (dbm.internals.matching) {
delete dbm.internals.matching;
}
if (dbm.internals.migrationMode) {
delete dbm.internals.migrationMode;
}
if (dbm.internals.argv.count) {
delete dbm.internals.argv.count;
}
if (dbm.internals.locTitle) {
delete dbm.internals.locTitle;
}
if (dbm.internals.migrationsDir) {
delete dbm.internals.migrationsDir;
}
}

public static resetInstanceAsync(dbm: any): Promise {
return new Promise((resolve, reject) => {
try {
DBMigrateHelper.resetInstance(dbm);
resolve();
} catch (e) {
reject(e);
}
});
}

public static executeUp(dbm: any, scope?: string): any {
return dbm.up(100, !scope ? '' : scope);
}

public static executeDown(dbm: any, scope?: string): any {
return dbm.down(100, !scope ? '' : scope);
}
}
```

---
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