bigtable: easier full table scans

# Issue

Currently, to do a full table scan (read all rows), you need to write code like so:

```
table := client.Open("mytable")

// This filter is a RowSet that specifies all the rows in the table
filter := bigtable.InfiniteRange(string([]byte{}))

ctx := context.Background()
t.ReadRows(ctx, filter, func(r bigtable.Row) {
// Do something with the returned rows
})
```

# Proposed solution

Add a method like `table.GetAllRows()` that would return an array of `bigtable.Row` objects, without needing to specify a filter. (Or even an `All` filter.)