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