Add support for mmap-backed CDB reader

[gnewton]

Hello,
I saw the issue on the https://github.com/colinmarc repo regarding using an mmap-backed reader colinmarc/cdb#11 and thought that it might be something to play with to see if it returns similar read performance improvements for cdb64. The performance improvements are pretty compelling (x35 faster). The 0 allocs/op is also nice. :-)

A fork of the colinmarc repo by the person who authored the above issue has implemented the mmap reader: https://github.com/cipriancraciun/go-cdb-lib

Thanks,
Glen