Use colon as scope separator throughout OTP to align with expected 1.0 API

The RouteMatcher should be the only place that still uses underscores, because it is
likely to be replaced entirely.

The reasoning for this whole group of changes is that colons are frequently used as scope separators,
underscores are often encountered in identifiers themselves which makes for confusion between the
separator and the ID, and finally we would like qualified GTFS entity IDs to look like URIs,
something like TriMet:route:15