MemcachedTTLConverter

Convert an optional Duration to an int suitable for passing to Memcached.

From the Memcached protocol spec:

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 606024*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.