Add HKP database backend

A potential use case of onak is as a proxy server. Add an HKP backend
that uses libcurl to make requests to a remote keyserver to fetch, search
or store keys. The "db_dir" configuration parameter becomes the base
host name for the remote keyserver e.g.:

db_backend hkp
db_dir the.earth.li

In the future the addition of the ability to stack database backends
should allow this to be used to turn onak into a caching keyserver.