Moved HttpRequest lock/link counter to the base HttpMsg class. This

Moved HttpRequest lock/link counter to the base HttpMsg class.  This
makes it easy to also lock-count HttpReply instances.

Removed HttpReply::swapOut(), which was a trivial wrapper around
StoreEntry::storeEntryReplaceObject().  Just call storeEntryReplaceObject()
instead.

Removed HttpReply::absorb() which was a hack to deal with the fact that
HttpReply's were not lock-counted.