obfs4: Alter tear down behavior to be less distinctive

The old behavior closed the connection on handshake failure after:
 * The first N bytes (random on a per-server basis).
 * The first M seconds (random on a per-server basis).

Whichever came first.  As Sergey Frolov kindly points out, depending on
which conditions cause termination, the server will send either a FIN or
a RST.  This change will remove the "amount read" based termination
threshold, so that connections that cause failed handshakes will discard
all data received until the teardown time is reached.

Thanks to Sergey Frolov for bringing this issue to my attention.