Cleanup transaction message size logging

Cleanup transaction message size logging

- Add class MessageCounters to track all size properties of a
  transaction message. HTTP-centric for now.

- replaced AccessLogEntry::cache.requestHeadersSize with
  AccessLogEntry::http.clientRequest.headerSz

- replaced AccessLogEntry::cache.requestSize with
  AccessLogEntry::http.clientRequest (total() of all counters)

- replaced AccessLogEntry::cache.replyHeadersSize with
  AccessLogEntry::http.adaptedReply.headerSz

- replaced AccessLogEntry::cache.replySize with
  AccessLogEntry::http.adaptedReply (total() of all counters)

- Rename the logformat byte codes for message size counters to
  form a logical set of details by message type

- reorganize and document logformat option codes more clearly

There are no logic changes in this patch. Several accounting
bugs have been identified and marked for fixing later.