Summary:
Refactor dump_tool and undump_tool so that it's possible to use them with customized options
for example setting a specific comparator similar to what Dragon is doing with the LdbTool

https://phabricator.fb.com/diffusion/FBCODE/browse/master/dragon/tools/Ldb.cpp

Test Plan:
compiles
used it to dump / undump a dragon shard

Reviewers: sdong, yhchiang, igor

Reviewed By: igor

Subscribers: dhruba, adsharma

Differential Revision: https://reviews.facebook.net/D47853