spanner: decoder should re-use arg arrays

[jba]

Currently, when the spanner client decodes an array column, it always creates a new array. This can result in more memory allocation than necessary. In this example, memory for a is reallocated for each row.

var a []string
... 
for {
    row, err := iter.Next()
    ...
    ... row.Column(0, &a) ...
}

This behavior is certainly safer than re-using memory, when the caller saves a for later use. But it makes it impossible to control memory. If we instead re-used the memory of a, then users could choose whether they wanted to save memory or make a copy.

If we do this, I recommend the following algorithm, which is consistent with the BigQuery client: if the given slice is too short (which includes the case where it is nil), allocate a new one. Otherwise, slice the given one to the right length and reuse it.

/cc @pongad @vkedia

[jba]

Moved to wiki.