Investigate the block counter size

[kitsook]

Properly handle 32-bit vs 64-bit block counter.

Quote from IETF draft:

Note also that the original ChaCha had a 64-bit nonce and 64-bit
block count. We have modified this here to be more consistent with
recommendations in section 3.2 of [RFC5116]. This limits the use of
a single (key,nonce) combination to 2^32 blocks, or 256 GB, but that
is enough for most uses. In cases where a single key is used by
multiple senders, it is important to make sure that they don't use
the same nonces. This can be assured by partitioning the nonce space
so that the first 32 bits are unique per sender, while the other 64
bits come from a counter.