DB Backend: convert TransactableHandle to an interface

Created by: camdencheek

This converts TransactableHandle to an interface and mints three new implementors of the interface: dbHandle, txHandle, and savepointHandle.

The general concept here is to create a TransactableHandle at startup, then it's a completely opaque type (not strictly yet, but that's the goal). Then, dbHandle.Transact() returns txHandle, txHandle.Transact() returns savepointHandle, and savepointHandle.Transact() returns another savepointHandle.

This design allows us to maintain concrete access to the underlying *sql.DB and sql.Tx handles while still providing the opportunity to mock the handle. Additionally, this design completely removes all the required interface casting, unwrapping, and mutation that has been a frequent cause of headaches for people looking at this code (me in particular).

Another thing this will enable is reporting when a transaction is used concurrently. Previously, we would get a random selection of conn busy, bad connection, or a panic that was very difficult to debug. This will allow me to lock a transaction during execution and report when a caller tries to use a transaction concurrently.

The new handle types are not constructible yet (unexported and not returned by any public methods). This is intentional because I want this PR to be generally about the design of the new handles and not about implementing their usage. I will migrate tests when I start using them.

Stacked on #37056

Test plan

The new types are unused. Depending on unit tests to verify the correctness of the gopls renames.