File expiration.

Expiration is currently riding on memcached's expiration model, so it's
a relative number of seconds, or an absolute in unix time as described
below:

    The actual value sent may either be Unix time (number of seconds
    since January 1, 1970, as a 32-bit value), or a number of seconds
    starting from current time. In the latter case, this number of
    seconds may not exceed 60*60*24*30 (number of seconds in 30 days);
    if the number sent by a client is larger than that, the server will
    consider it to be real Unix time value rather than an offset from
    current time.

cbugg: close bug-314