[JUJU-3907] Rename sqlair.TX and associations to use "Tx"

[manadart]

Here we rename the sqlair typeTX to Tx, along with associated methods/types.

This is for:

• Idiomatic Go (abbreviation, not acronym).
• Congruence with sql.Tx from the standard library.

[niemeyer]

We've discussed this before, and the rationale behind the change is that both DB and TX have almost exactly the same API, so writing code for both made it obvious that there's an inconsistency, and we've decided to fix it on the SQLair layer. Happy to have Db and Tx instead if that sounds better? ;-)