que_jobs table will be locked from updates during migrations' index creation

[ZimbiX]

It's just occurred to me that the index creation on que_jobs in the migrations for v1.3.0 (#334) and v2 (#319) will lock the que_jobs table from updates for the period of creating the index. This could be a problematically long time for people who have a big que_jobs table.

We can't just change them to use CREATE INDEX CONCURRENTLY (CIC), since a migration runs in a transaction, and CIC doesn't work in a transaction. We could modify them to use IF NOT EXISTS, so if needed, someone could manually run a CIC version of the index creation before applying the migration.