Mattermost-server: mattermod: move away from gorp to sqlx in pull_request_store.go

[mattermod]

We are planning to move away from gorp to using sqlx in our server repo to perform any SQL queries. And doing that change in mattermod is the first step in getting some experience in doing that.

Mattermod is a relatively small codebase with just 3 tables.

This PR is about migrating the pullrequeststore.go to use sqlx. Here is a dummy PR https://github.com/mattermost/mattermost-mattermod/pull/241 which gives an outline of how to do that.

Once you have done that, your local unit tests should pass. That is enough indication that the migration has been done. There is NO need to run and test mattermod end to end for this.

Essentially, we need to move away from using gorp methods to using sqlx methods.

If you have questions about the ticket or you need any help, feel free to contact agniva.de.sarker in https://community.mattermost.com/

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JIRA: https://mattermost.atlassian.net/browse/MM-28977

[rvillablanca]

I release this issue since I haven't had time to work on this.

[agnivade]

@kashifsoofi - Wondering if you are up for this and drive home a clean run finishing all the stores in mattermod? :)

[kashifsoofi]

@kashifsoofi - Wondering if you are up for this and drive home a clean run finishing all the stores in mattermod? :)

Sounds good, i can take this.