Provide an access to a document info

The functions `get_doc_info/3` and `get_full_doc_info/3` were
added to API to provide an access to the records `#doc_info{}`
and `#full_doc_info{}` accordingly.

The functions are re-using `fabric_open_doc` coordinator
and consequently are the subject of the same read quorum rules
as `open_doc/3` function. However the info functions do not trigger
read repair on a not fully complete quorum.

Function `get_full_doc_info/3` accepts an option `deleted` to
allow to provide an information for a deleted document,
similar to `open_doc/3`.

FogBugz: 12933
Merge pull request #157 from cloudant/12933-access-to-full-doc-info

Provide an access to a document info