Remove cursor.rewind interface

Our cursor interface has no nextObject method, so there's no point in
having a rewind method. Its major effect is ensuring that
fetch/forEach/map return no documents if you've already called one of
them once. It's not clear why this is actually useful to anybody.

rewind is kept around as a no-op; if we later implement nextObject, we
can make rewind do something, but we still presumably would auto-rewind
before fetch/forEach/map.

In minimongo, remove the db_objects cache inside each cursor. The only
actual use of this cache was that if you called count multiple times, it
would return the same number without re-running the query, and you could
share the query work between N calls to count and one call to
fetch/forEach/map (but only one call! future calls would return
nothing!)  While there's a minor performance hit from getting rid of
this cache, it should also use a little less memory, and enable use
cases like

   {{#with someCursor}}
     {{#if count}}
        {{#each this}}
          ...
        {{/each}}
     {{/if}}
   {{/with}}

which didn't work before because even the deps invalidation didn't
rewind the cursor.

Also, as a minor optimization, skip an EJSON.clone if there's a
projection, because projection functions are guaranteed to clone.

Fixes #2114
Only do one query for forEach/count in Deps

They used to run the query twice: once for the actual result and once to
set up the observe. Now it shares the work between the observe and the
actual query.

This required me to inline the _depend helper, but I actually think this
made what's going on more direct and clear.

Drop the _allow_unordered hack. I'm not convinced that it was ever truly
valid; the observe code really doesn't support unordered observes with
skip and limit, and I could not remember what it was about count's use
that made it hypothetically safe.  Easier to just remove the hack (until
we maybe eventually actually fix #1643)

Stop using Deps.Dependency in an unidiomatic way; just use
Deps.currentComputation directly.