TS-2507: fix the state transition logging for handle_server_setup_error

STATE_ENTER should take a VC event code, not a HttpApiState_t. Also,
STATE_ENTER is conventionally called on entry to the state handler,
not somewhere in the middle.
TS-2508: escalation plugin

The escalation plugin is intended to allow Traffic Server to handle
origin errors by rerouting to a different URL. This doesn't currently
work, since the APIs it uses cause crashes. In it's current form,
it should be useful as a test case to work on fixing the APIs.
TS-2509: add const qualifier to pure HttpTunnel member functions