This reverts the core change from: https://github.com/capnproto/capnproto/commit/6e4c5ce3c9f31a5e79b50cd9eaae6d03106edf98

There are a few reasons to believe that change was wrong:
- It meant that a pump would always throw and catch a DISCONNECTED exception, which violates the rule that we don't throw exceptions in "normal" operation.
- It could lead to trouble when combined with the previous commit, which could delay WebSocket disconnect until the service "completes" -- which would lead to a hang.

The commit message where I introduced this said something about bad behavior on OSX. We should address that directly, if it is still a problem.