Change primary/secondary server list to priority list

[darkain]

Allow unlimited number of server priority levels.

eg:

1: db1 1: db2 2: db3 3: db4 4: db5

db1 and db2 share a priority. if both are unavailable, db3 is used. if that is unavailable, db4 and db5 share a priority.

helpful for database locality. an example: 1: same availability zone 2: same dc, different availability zone 3: different dc, same coastal region 4: different dc, same continent 5: different continent