Add Len() method to *pgx.Batch

[WGH-]

This makes the API slightly easier to use when number of calls to
Queue() cannot be trivially computed.

For example, if the program contains the loop like the following,
a separate variable counting the iterations is needed:

numHeaders := 0
for _, header := range prepareHeadersForInsert(*res.Headers) {
    headerBatch.Queue("INSERT ...", ...)
    numHeaders++
}

headerBatchResult := tx.SendBatch(ctx, headerBatch)

for i := 0; i < numHeaders; i++ {
    _, err := headerBatchResult.Exec()
    // ...
}

With method Len(), this extra variable can be eliminated.

[jackc]

Thanks!