Switch to non-closure based transactions

[fishcakez]

Requiring a fun for a transaction and non-local return for a rollback is a very limiting API. With this breaking change the API becomes more flexible, in particular it allows transaction to continue over multiple OTP behaviour callbacks and use of (potentially nested) savepoints.

Removes:

• DBConnection.transaction/3
• DBConnection.rollback/2

Adds:

• DBConnection.checkout/2
• DBConnection.checkin/2
• DBConnection.begin/2
• DBConnection.commit/2
• DBConnection.rollback/2
• DBConnection.checkout_begin/2
• DBConnection.commit_checkin/2
• DBConnection.rollback_checkin/2

[josevalim]

Do we need all 8 new functions? I assume the last three are for convenience?

[fishcakez]

Closed in a favour of #88 #89 #90