Turn on client iat-mode for some default obfs4 bridges

Since the last release, some of the default bridges have turned on obfs4's packet size and timing obfuscation (iat-mode). Here is the thread where it came up: [tor-project] Turning on timing obfuscation (iat-mode=1) for some default bridges https://lists.torproject.org/pipermail/tor-project/2016-November/000776.html

We should make the client setting of iat-mode match the server setting. (If the two sides do not have the same setting, the connection will still work, but only one direction will be using the packet size and timing obfuscation.)