Avoid allocating on each call to rows.Columns

[nussjustin]

Description

Calling rows.Columns() always allocates a new slice, even if it was already called once. Cache the slice on the first call and return the cached value on subsequent calls.

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[methane]

LGTM

Also if we have a prepared statement, cache the column names in the statement the first time it gets executed and set them the next time the prepared statement gets executed (as is already done with the column information).

I'm afraid that adding / removing column from another session may break the cache.
But column info caching is already there. This PR doesn't make it worse.

[nussjustin]

Okay, wow. Today I learned that a wrong push closes a PR Oo

[julienschmidt]

You can reopen it after making another push to the branch (or just rebase and force-push)

[julienschmidt]

Or at least team members can reopen it then

[nussjustin]

That worked :-) I also updated the benchmark in the PR for the Go 1.8 beta.

[methane]

Please merge master.

Also if we have a prepared statement, cache the column names in the statement the first time it gets executed and set them the next time the prepared statement gets executed (as is already done with the column information).

The information cache is removed because prepared statement can return different columns.

[coveralls]

Coverage increased (+0.04%) to 74.573% when pulling 5cd094e on nussjustin:master into aeb7d3c on go-sql-driver:master.

[julienschmidt]

Thanks for your first contribution to the project! 👍