Deduplicate transaction LogTags (#906)

Deduplicate transaction LogTags (#906)

...using ALE cache.code instead of its partial
ClientSideRequest::logType duplicate.

The need to de-duplicate these two LogTag members became apparent when
we tried to update/detail LogTags in forwarding code (e.g., to report
peer timeouts) and noticed that ALE cache.code value could be
overwritten by (stale) ClientSideRequest::logType information. Those
updates needed other significant changes, so we factored this small
stand-alone improvement out.

Fortunately, ClientSideRequest::logType lifetime does not exceed ALE's,
making ClientHttpRequest::logTags removal straightforward.

Also removed a paranoid TunnelStateData::al check after verifying that
the ALE pointer is never nil.