Map downloads: one that has begun goes on when the session stops waiting

Follow-up to #225. The session still waits 30 minutes in all for a map
(checking, the two-minute question, download, install, apply, join), and
servers and lobbies still let a player go at 30 minutes. But a download
that has begun is no longer cut off at 27 minutes: a large map on a slow
connection never came at all that way.

- Once the download has begun, the session's deadline is the session's
  alone. The download goes on (up to its own four hours, as before #225),
  installs and is applied, and the player is told to join again; no
  rejoin is tried past the deadline, where the server would turn them
  away. "Map still downloading" says so when the session stops waiting.
- Before the download, the deadline still ends the check and the question.
  Declining or cancelling still stops it and removes the partial file.
- A download whose session has gone is not cancelled by joining another
  session; a new map need is turned away until it is done.
- The 27-minute transfer budget and its completion reserve are gone from
  map_fetch_policy.h, with their tests; the map download tests cover the
  new behaviour.
- The HTTPS timeout test compiles with warnings as errors (a signed and
  unsigned comparison), and get_text's stale-file removal is indented again.
- README: map downloads described as they now are.

Engine/Vfs/Test/https_download_timeout_tests.cpp

@@ -253,7 +253,7 @@ int main(int argc, char** argv) {
             fake_http::state.stop_at = stage;
             fake_http::state.timeout = timeout;
             result = get();
-            check(!result.ok && result.error == (timeout ? ERROR_TIMEOUT : ERROR_CANCELLED),
+            check(!result.ok && result.error == static_cast<DWORD>(timeout ? ERROR_TIMEOUT : ERROR_CANCELLED),
                   "pending operation or final completion reports the authoritative stop reason");
             check(removed(), "stopped GET removes only its created output, including partial bytes");
             check(closed(), "stopped request completes handle-closing lifetime handshake");