Kill the session after encrypting 2^47 blocks

"Both the privacy and the authenticity properties of OCB degrade as
 per s^2 / 2^128, where s is the total number of blocks that the
 adversary acquires.... In order to ensure that s^2 / 2^128 remains
 small, a given key should be used to encrypt at most 2^48 blocks (2^55
 bits or 4 petabytes)"

-- http://tools.ietf.org/html/draft-krovetz-ocb-03

We deem it unlikely that a legitimate user will send 4 PB through a Mosh
session.  If it happens, we simply kill the session.  The server and
client use the same key, so we actually need to die after 2^47 blocks.

Closes #77.