river_job.max_attempts (the database column) should have a default value of 25 #1012

While writing documentation for how to insert a job using raw SQL, I came across the fact that while almost every river_job field gets a reasonable default, max_attempts is the odd man out and must have a value specified despite having a clear default of 25 in code.

Having a default on this column would be a slight improvement in that it'd make a manually inserted job a little easier to insert. Also, I think not having a default on this property may have been a mistake anyway since all other properties except the highly variable ones like kind and args have one.

Not worth its own migration, but let's ship the change opportunistically during the next migration that goes out.