Provide an access to a document info

The coordinator fabric_open_doc was modified to accept an option
`doc_info` or `{doc_info, full}` to provide an access to a record
`#doc_info{}` or `#full_doc_info{}` of a document accordingly.

The same functionality was exposed as a public API functions
get_doc_info/3 and get_full_doc_info/3 to match couch_db API

A query for a document info is the subject of the same read quorum
rules as the open_doc/3 function.

An option `deleted` could be applied to get_full_doc_info/3 call
to give an access to a full info of the deleted document in a manner
similar to open_doc/3.

FogBugz: [12933](https://cloudant.fogbugz.com/f/cases/12933/Provide-access-to-the-full_doc_info-from-fabric)
changing this record will crash in-flight fabric_doc_open calls during upgrades? (cc @davisp)