Add connection ID to ORCONN event.

Jointly authored with Rob Jansen <jansen@cs.umn.edu>.
Add new CONN_BW event.

Jointly authored with Rob Jansen <jansen@cs.umn.edu>.
Add new CELL_STATS event.

Jointly authored with Rob Jansen <jansen@cs.umn.edu>.
Add new TB_EMPTY event.

Jointly authored with Rob Jansen <jansen@cs.umn.edu>.
Add new CIRC_BW event.
Tweak ORCONN event based on comments by nickm.

- Move new ID= parameter in ORCONN event to end.  Avoids possible trouble
  from controllers that parse parameters by position, even though they
  shouldn't.
Tweak CONN_BW event based on comments by nickm.

- Rename read/write counters in connection_t to make it clear that these
  are only used for CONN_BW events.
- Add TestingEnableConnBwEvent option.
Tweak CELL_STATS event based on comments by nickm.

- Move cell_command_to_string from control.c to command.c.
- Use accessor for global_circuitlist instead of extern.
- Add a struct for cell statistics by command instead of six arrays.
- Split up control_event_circuit_cell_stats by using two helper functions.
- Add TestingEnableCellStatsEvent option.
- Prepare functions for testing.
- Rename a few variables and document a few things better.
Tweak TB_EMPTY event based on comments by nickm.

- Avoid control_event_refill_global function with 13 arguments and
  increase code reuse factor by moving more code from control.c to
  connection.c.
- Avoid an unsafe uint32_t -> int cast.
- Add TestingEnableTbEmptyEvent option.
- Prepare functions for testing.
- Rename a few functions and improve documentation.
Tweak CIRC_BW event based on comments by nickm.

- Rename n_read and n_written in origin_circuit_t to make it clear that
  these are only used for CIRC_BW events.
- Extract new code in control_update_global_event_mask to new
  clear_circ_bw_fields function.
Test functions used for TB_EMPTY and CELL_STATS events.
Don't access global_circuitlist variable directly. Use a getter instead.
Fix #9108 - make global_circuitlist a doubly linked list
Repair of some of the lost parameter filters history
Added getter for protected parameter
git test..
(undo) git test..
Skeleton ExtORPort implementation.  Needs testing, documentation.

Does not implement TransportControlPort yet.
Document code and change the Extended ORPort command numbers.
Create the Extended ORPort authentication cookie file.