The problem: WHEREIS must come after PREPARE, and external remotes are
encouraged to establish network connections in PREPARE to simplify their
implementations. So `git annex whereis` with such an external remote
enabled and content on that remote is slowed down by an unnecessary network
connection.

Note that this even happens when the external remote does not implement
support for WHEREIS.

So, we want to deprecate WHEREIS needing to come after PREPARE.

GETINFO has the same problem. (GETCOST and GETAVAILABILITY also, but
those are cached to avoid needing to be run more than once per remote.)

The transition: 

* Add an extension to the protocol that lets WHEREIS come before/without
  PREPARE. The external remote can indicate it supports this by negotiating
  EXTENSIONS WHEREIS.

  And similarly for GETINFO, with EXTENSIONS GETINFO.

* For a while, git-annex can continue to send WHEREIS after PREPARE when
  the external remote doesn't support the protocol extension. If the
  external remote replies with WHEREIS-SUCCESS, then git-annex can warn
  that it needs to be updated for the transition. (If it replies with
  WHEREIS-FAILURE, it may be that it only has stub whereis support that
  doesn't do anything, so probably git-annex should avoid warning in that
  case).

  And similarly for GETINFO, warning when it is sent after PREPARE and
  there are any INFOFIELD replies, but not warning if there's only an
  INFOEND.

* Eventually, git-annex can stop sending WHEREIS unless the protocol
  extension was negotiated. So WHEREIS stops being part of the
  base protocol and becomes an extension.

  Ditto for GETINFO.

Possible complications:

An external remote may need to do some kind of expensive setup to prepare
to reply to WHEREIS, such as querying with GETCONFIG or looking at its own
internal state or whatever. With WHEREIS run before PREPARE, such a remote
will need to defer that setup until the first WHEREIS and cache it for
subsequent WHEREIS, which may complicate its code slightly.

Note that the protocol does allow querying with GETCONFIG etc before
responding to a WHEREIS request.
