PublisherConcatMapIterable simplify control flow

`PublisherConcatMapIterable` has multiple embedded try/finally blocks which makes control flow difficult to reason about. Some code in a `finally` blocks which calls out to user code which may throw, further complicating control flow.

(original discussion https://github.com/apple/servicetalk/pull/1026#discussion_r416067011)

Start by locating PublisherConcatMapIterable and reading its embedded try/finally blocks, then review the original discussion in pull request #1026. Done means the control flow is easier to reason about while preserving behavior, especially when user code called from cleanup throws.

Written by the indexing model from the issue text.