obfsproxy: Don't read authcookie file for every new connection
Currently, obfsproxy opens the authcookie file for each new incoming connection: https://gitweb.torproject.org/pluggable-transports/obfsproxy.git/blob/HEAD:/obfsproxy/network/extended_orport.py#l250
This is suboptimal, and we can just read the file on startup and keep the authcookie value around during runtime.
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