Add a fast FIFO buffer implementation.

The implementation is faster than using a simple string as network buffer
(which is copied for every write).
Use the FIFO buffer instead of a string buffer.

The FIFO buffer is not copied for every write and as a result significantly
faster.  That should only matter if ScrambleSuit is ran over LAN-speed
networks.
Add an option to weaken obfuscation.

Add the constant 'USE_IAT_OBFUSCATION' which, when set to 'False', disables
inter-arrival time obfuscation.  This will make the transported protocol more
identifiable but increases throughput a lot.
Consider 'USE_IAT_OBFUSCATION' when sending data.

If 'USE_IAT_OBFUSCATION' is set, data is written immediately without
considering inter-arrival time obfuscation.