[server] Avoid locking databases during backup
[server] Avoid locking databases during backup

Summary: Since all of our tables are InnoDB, we can use `--single-transaction`, which will disable `--lock-tables` and allow MySQL to continue processing while the backup is ongoing.

Test Plan: I ran the `mysqldump` command on the server with the specified parameters and made sure I could still load the website while the backup was occurring

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: KatPo, zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D237

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