The concurrent-ruby gem is a toolset containing many concurrency
utilities. Many of these utilities include runtime-specific
optimizations when possible. Rather than clutter the Rails codebase with
concurrency utilities separate from the core task, such tools can be
superseded by similar tools in the more specialized gem. This commit
replaces `ActiveSupport::Concurrency::Latch` with
`Concurrent::CountDownLatch`, which is functionally equivalent.
avoid freezing the headers until the web server has actually read data
from the body proxy. Once the webserver has read data, then we should
throw an error if someone tries to set a header
when streaming responses, we need to make sure the cookie jar is written
to the headers before returning up the stack. This commit introduces a
new method on the response object that writes the cookie jar to the
headers as the response is committed. The middleware and test framework
will not write the cookie headers if the response has already been
committed.
fixes#14352
