mmr_bench with any key_size.

Allow mmr_bench to be used with different key sizes, configurable by the
parameter in the top called key_size. We have set it to 36 so this benchmark
can be compared against htdb_record_bench. I'm getting a multimap speed that is
7.5x slower than the single map (where each entry of the multimap has between 1-10
random records).
simple typo in comment