Increase MIN_BUFFER_SIZE to 512 (1024 on 64-bit)

Increase MIN_BUFFER_SIZE to 512 (1024 on 64-bit)

This constant decides the smallest (and typical) size of each evbuffer
chain.  Since this number includes sizeof(evbuffer_chain) overhead,
the old value (256) was just too low: on 64-bit platforms, it would
spend nearly 20% of the allocations on overhead.  The new values mean
that we'll be spending closer to 5% of evbuffer allocations on overhead.

It would be nice to get this number even lower if we can.