Implement Prop#324 Flow Control

There's enough pieces of Flow Control that need to be done to get it merge-ready that a checklist and ticket seem wise at this point.

Here's stuff I will do:

• Improve monotime checks to export a global monotime status
• Remove mobile xon/xoff limits
• Advertise the average and max edge conn drain rates in XON (after first XOFF, or if above a "low watermark" queue length)
• Low watermark params to send advisory XON before XOFF
• Send periodic XONs if the drain rate changes significantly
• Perform checks for semantically valid XON/XOFF and call circuit_read_valid_data()
• Turn CELL_QUEUE_HIGHWATER_SIZE (and others?) into consensus params
• Implement ways to restrict when advisory XONs can be sent, to reduce side channels from exits
• Alter half-open edge connection checks to work with XON/XOFF wrt valid data
• Fix edge case where XON/XOFF can arrive after stream close (depends on half-open fix)
• Additional consensus parameter for CircEWMA's EWMA_TICK_LEN and edge ratelimit low/high change
• Preliminary tuning of new consensus params over onion svcs
• Misc XXX's
• Update Prop#324 spec with above
• Debug log removal and other log message cleanups/improvements
• Squash branch for review

Here's stuff @dgoulet can do:

• Preliminary code review
• Rate limit packaging data on edge connection's circuits (or reading on edge source sockets?) to match the advertised rate using token buckets
• Improve oomkiller/circuit closing wrt total edge connection outbuf lengths on circs
• Determine if we can use any kernel info from KIST to improve buffer length calls (there's some XXX's to note where in the flow control code this may help)
• Spotcheck existing oomkiller, KIST, and CircEWMA code to see if we should parameterize or tighten anything else for congestion control generally
• Determine if we should parameterize any other buffer lengths inside channel handling, cell handling, and the circuitmux dragon
• Help me figure out why flow_control_decide_xon() is almost always only called when the outbuf is 0, where the data is going to, and what we should do about it (KIST, ask the socket if it wouldblock?)
• Test out the flow control branch on some rate limited onions for weirdness. Help decide some tuning parameter values.

Cc: @dgoulet