Allow partial pipelining within a transaction

Allow pipelining the some requests in a transaction without requiring that all the requests are pipelined. This would allow selecting to get the id of a row, then issuing multiple statements based on the id of the row in a pipelined fashion.

For example:

await sql.begin(async sql => {
  const [movie] = await sql`
    SELECT * FROM Movie
    WHERE name = 'Star Wars'
  `;
  return [
    sql`
      INSERT INTO MovieActor
      -- some statement using ${movie.id}
    `,
    sql`
      INSERT INTO MovieReleases
      -- some statement using ${movie.id}
    `,
     // etc
  ]
})

Start at the transaction entry point shown in the issue, sql.begin, and trace how tagged sql requests are currently grouped and executed. Compare the existing all-pipelined behavior with the requested mix of an awaited result followed by pipelined statements; done means both sequential and pipelined requests work within one transaction.