AUTH support fails on databases beyond 0

[fgm]

Cause: when Redigo connects to a Redis server,

• if the requested DB is 0, it only connects, and the AUTH command is applied when building the AuthConn afterwards
• if the requested DB is not 0, it first connects, then uses SELECT, which fails before returning, before the AuthConn is built

Solution:

• either modify the DSN in all cases based on the -u/-p flags to include auth, allowing the removal of the AuthConn
• or modify the DSN to connect to DB 0, build the AuthConn, and SELECT the requested DB afterwards.

The first approach seems more elegant, albeit a bit more fragile as it depends on an undocumented behavior of Redigo.

[fgm]

Underlying issue in the way Redigo handles DSNs: gomodule/redigo#561

[fgm]

Fixed by gomodule/redigo#566