Added data packet padding to toxcore.

Data sent as lossless or lossy is now padded with:
((MAX_CRYPTO_DATA_SIZE - data_length) % CRYPTO_MAX_PADDING) bytes
in order to reduce the possibility of length related attacks.

I set CRYPTO_MAX_PADDING to 8 but it can be changed anytime without
breaking network compatibility between tox cores.
Request packets are now sent with send_data_packet_helper().

This means all data packets are now padded the same way.
Doubled minimum transfer rate.

File transfers got stuck because it was too low.