Fix deadlocks in Retry and FollowRedirect middlewares

[rossabaker]

These should never allocate a second connection before releasing the first. If they do, we can deadlock trying to allocate a second connection while everybody holds onto their first.

Fixes #5180.

[rossabaker]

The fact we had to invoke both operations of Hotswap suggests to me that this just isn't the right state machine, but this solution seems to work. Maybe I can nerdsnipe @ChristopherDavenport into a better state machine for this purpose.

[rossabaker]

This is a good diff to review with whitespace insignificance.