Reduce crashes due to unexpected ClientHttpRequest termination.

Reduce crashes due to unexpected ClientHttpRequest termination.

The underlying problem has been known since r13480: If a
ClientHttpRequest job ends without Http::Stream (and ConnStateData)
knowledge, then Squid is likely to segfault or assert. This patch does
not resolve the underlying issue (a proper fix would require
architectural changes in a consensus-lacking area) but makes an
unexpected ClientHttpRequest job destruction less likely.

BodyPipe and Adaptation-related exceptions are the major causes of
unexpected ClientHttpRequest job destruction. This patch handles them by
closing the client connection. Connection closure should trigger an
orderly top-down cleanup, including Http::Stream, ConnStateData, and
ClientHttpRequest destruction.

If there is no connection to close, then the exception is essentially
ignored with a level-1 error message disclosing the problem. The side
effects of ignoring such exceptions are unknown, but without a client
connection, it is our hope that they would be relatively benign.

This is a Measurement Factory project.