fix: disable migration lock by default on CheckRepoStatus

[dkarter]

Motivation

The default behavior of using locks when checking migration status caused a hard
to debug bug when migration_lock: :pg_advisory_lock was set on the repo config.

The bug was causing multiple slow downs on web requests due to attempts to acquire locks
on the database.

Generally locks should not be necessary for the pending migration check since we
are just reading from the migrations table and not actually running the migrations.

Note

I wasn't sure if we should just deprecate the option altogether. For now that means that users can still enable migration_lock if they find a need for it - I'm not sure if that need really exists. Open to change that.

Changes

Made :migration_lock option on CheckRepoStatus default to true.

[josevalim]

💚 💙 💜 💛 ❤️

[josevalim]

Please give it a try for a while in your app and let us know if it all goes well!

[cgrothaus]

Oh, this is cool 😍 ! Thanks @dkarter

I once tried to solve this in the phx_new template, to put the migration_lock: false option in the generated code: phoenixframework/phoenix#5153 But that PR got rejected 🤷 .

Your approach is smarter, and it's cool that it got accepted!