Fix misuse of sync.Pool

[darhwa]

1. What does this change do, exactly?

The buffers got from bufferPool are never put back. They will still get GCed sometime later, but that's not the purpose of using sync.Pool

2. Please link to the relevant issues.

N/A

3. Which documentation changes (if any) need to be made because of this PR?

None.

4. Checklist

• I have written tests and verified that they fail without my change
• I made pull request as minimal and simple as possible. If change is not small or additional dependencies are required, I opened an issue to propose and discuss the design first
• I have squashed any insignificant commits
• This change has comments for package types, values, functions, and non-obvious lines of code

[mholt]

Thanks for the PR!

Can you actually make this against the caddy2 branch, here: #74 ? Because the v1 / master version is no longer being maintained at this point, I guess.

[darhwa]

@mholt

Thanks for the prompt reply!

I've created a new one (#76) against caddy2. Think this can be closed.