Lazy transaction BEGIN

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**Is your feature request related to a problem? Please describe.**
On a high latency connections every additional roundtrip between client and server is undesirable. `Conn.BeginTx` starts transaction by `Conn.Exec`ing `BEGIN` command, which internally waits for result. This wait time is a wasteful and app can't do much about it.

**Describe the solution you'd like**
Delay `BEGIN` and sendit it together with first query in transaction, essentially making it lazy. Alternatively utilize Pipeline/Batch to send it in `BeginTx`, but don't wait for it's result until first query in transaction is issued.

**Describe alternatives you've considered**
App can chose to use Batch and manually roll `BEGIN`/`COMMIT` into the batch, but then it loses convenience of `Tx` API.

Start at Conn.BeginTx and the Conn.Exec path that sends BEGIN. Compare the proposed lazy behavior with Pipeline/Batch and the existing Tx API. Done means BEGIN is sent with the first transaction query, or its result is deferred until then, while preserving the Tx API.

Written by the indexing model from the issue text.