helper: remove timings["existence_check"], aka "Already-In-Grid Check"

This measured how long the Helper took to do a filecheck before asking
for ciphertext. The "Contacting Helper" report includes both
existence_check and the client-helper RTT.

For non-overlapping uploads, it was being returned correctly. But when
multiple upload requests overlapped, and the file was not already in the
grid, the filecheck would only run once, and its existence_check time
would be reported for all uploaders (even if they didn't have to wait
for that time). Cleaning that up proved too difficult: the only correct
place to report this time is from the initial remote_upload_chk() call,
but the return value of that is too constrained to accomodate it in the
needs-upload case.

So I'm removing it altogether. Eventually I plan to add a proper
events/times field and record more data, including this check, in a form
that can be drawn on a nice zoomable timeline view.

Old clients talking to a new Helper (which doesn't supply the value)
will tolerate the loss (they'll just display an empty field on the web
view).
clean up Helper to make later changes easier

Fix up control flow inside the Helper, to make it more friendly for
later refactoring.
Uploader cleanup: create results at end, not beginning

This will make it easier to populate the UploadResults during __init__,
instead of doing it one-field-at-a-time later.
add HelperUploadResults

This splits the pb.Copyable on-wire object (HelperUploadResults) out
from the local results object (UploadResults). To maintain compatibility
with older Helpers, we have to leave pb.Copyable classes alone and
unmodified, but we want to change UploadResults to use IServers instead
of serverids. So by using a different class on the wire, and translating
to/from it on either end, we can accomplish both.
convert UploadResults to a fat init

Populate most of UploadResults (except .uri, which is learned later when
using a Helper) in the constructor, instead of allowing creators to
write to attributes later. This will help isolate the fields that we
want to change to use IServers.
switch UploadResults to use getters, hide internal data, for all but .uri

This hides attributes with e.g. _sharemap, and creates getters like
get_sharemap() to access them, for every field except .uri . This will
make it easier to modify the internal representation of .sharemap
without requiring callers to adjust quite yet.

".uri" has so many users that it seemed better to update it in a
subsequent patch.
switch UploadResults to use get_uri(), hide internal ._uri

Complete the getter-based transformation, by hiding ".uri" and updating
callers to use get_uri(). Also don't set a dummy self._uri, leave it
undefined until someone calls set_uri().
split IDisplayableServer from IServer, add sb.get_stub_server()

IDisplayableServer includes just enough functionality to call
.get_name() and friends, which is all that the UploadResults really
need. IServer is a superset that includes actual share-manipulation
methods. StubServer instances provide only IDisplayableServer, while
actual NativeStorageServer instances provide the full IServer interface.

When the Helper sends a serverid (so we know what to call the server but
nothing else about it, and have no corresponding NativeStorageServer
object to reference), but we want to store an IDisplayableServer in the
UploadResults, we create a synthetic StubServer "server" and store that
instead.
UploadResults: store IServers internally, but still return serverids

This stores IDisplayableServer-providing instances (StubServers or
NativeStorageServers) in the .servermap and .sharemap dictionaries. But
get_servermap()/get_sharemap() still return data structures with
serverids, not IServers, by translating their data on the way out. This
lets us put off changing the callers for a little bit longer.
change UploadResults to return IServers, update users to match

This finally changes all callers of get_servermap()/get_sharemap() to
accept IServers, and changes UploadResults to provide them.