Fix messages leaking from streaming HTTP responses

When we cancel a streaming HTTP response (ie, Transfer-Encoding:
chunked) its possible to leave ibrowse messages in the mailbox. For
processes like the changes reader that will continually retry HTTP
requests these messages can build up and cause the process to slow to a
crawl which in turns causes replications to seemingly get stuck.

This patch changes the HTTP request functions to be wholly contained in
a try/catch/after clause. Using the after clause we can be sure that
we're removing all ibrowse related messages before returning to the
calling function. We also release the worker from the same after clause
and inform the release if its a changes feed worker which is important
as these workers are not managed by a pool.

BugzId: 25341
Merge pull request #45 from cloudant/25341-message-leaks

Fix messages leaking from streaming HTTP responses