Undocumented CIRC closure reason

The 'MEASUREMENT_EXPIRED' reason in CIRC events and 'GETINFO circuit-status' is undocumented...

https://gitweb.torproject.org/torspec.git/blob/HEAD:/control-spec.txt#l1276

atagar@morrigan:~/Desktop/tor/torspec$ grep -R "MEASUREMENT_EXPIRED" ./*
./control-spec.txt:               "MEASUREMENT_EXPIRED"

This reason was added by Mike in 73f53343 so assigning to him...

https://gitweb.torproject.org/torspec.git/commitdiff/73f53343