support building w/ mongo-c-driver >= 2.0

[devjunix]

please apply the attached patch to support building w/ mongo-c-driver >= 2.0

hydra_mongo-c-driver2.patch

[vanhauser-thc]

you changes to hydra.c and hydra-mongo.c are correct, however i configure you completely remove mongo-1 support.
and please create a PR, your come up as a contributor, easier to review and communicate.