Unset draining after we are drained

[nullstyle]

No description provided.

[elliotf]

This doesn't seem to address the problem of bookshelf/knex having issues with disconnected DB connections. This change seems to only impact the caller when .drain() is called. Without this change, you need to create a new generic pool (which knex does) .. With this change, you could reuse a pool.

Is that what you were trying to achieve, @nullstyle ?