Http Common Duration Downstream Connection

Added support for the DS_CX_BEG and DS_CX_END :ref:COMMON_DURATION <config_access_log_format_common_duration> time points for HTTP. DS_CX_BEG reflects the downstream connection begin and repeats for all requests on a connection, while DS_CX_END is populated for requests that are active when the downstream connection closes and otherwise renders as "-".