Create Tx and Conn interfaces to allow prepared statements to be used in transactions

[RangelReale]

sql.DB allows creating a Statement using DB.Prepare, and later this prepared statement can be used inside a transaction using tx.Stmt(). For this to work, both the statement and the transaction must be using the same underlining *sql.DB.

This library returns a raw *sql.Tx for transactions, but a custom Stmt for Prepare, which can't be used with tx.Stmt().

This MR creates Tx and Conn interfaces matching database/sql, and makes sure that transactions and statements works the same as the default ones.

[bxcodec]

LGTM

@RangelReale can u help to test it first from master after merged?

[bxcodec]

Let's wait for the CI-test result before merging

[bxcodec]

LGTM