Move `Concentrate.Filter.GTFS.*` to `Concentrate.GTFS.*`.

We are introducing a `Concentrate.GroupFilter` that will use `GTFS`
data, so organizing this part of the app under the `Filter` namespace
doesn't make as much sense as it did before.

Building the records to be inserted into ETS can take a while (over a
minute), and since all records are deleted from the table before this
work begins, the module will return `:unknown` for all lookups during
this time. This issue would occur once per hour, as per the static GTFS
refresh interval, and would result in `RemoveUnneededTimes` temporarily
not working.

This refactor applies the same approach used in `StopIDs`: build all
records up-front, then clear the table and immediately insert all the
records in one batch. This still takes a second or two, but the time
gap is much shorter, reducing the chance that an incorrect feed would
be produced and the time until a corrected feed would be produced.

Eliminating the issue entirely will require a more complex approach,
such as writing some "upsert" logic which also deletes records that
shouldn't be in the table anymore, or using Mnesia for its transaction
support.

Having three different servers loading `stop_times.txt` and maintaining
their own subset of it was resulting in very high CPU and memory usage.
Combining these, similar to `GTFS.Trips`, saves on system resources and
significantly reduces the "warm-up time" in which filters are not fully
functional.

Approximate numbers on a developer machine:

Metric       | pre `StopTimes` | post `StopTimes` | consolidated
------------ | --------------- | ---------------- | ------------
Update Time  | 1.5 min         | 4 min            | 1.5 min
Peak Memory  | 2.2GB           | 3.3GB            | 2.0GB
Steady-State | 1.3GB           | 1.7GB            | 880MB

The first column is from when there were only two `stop_times.txt`
servers, `PickupDropOff` and `StopIDs`. We can see that consolidation
is also an improvement over these numbers.

There are some incidental changes that should not have any effect on the
behavior of the app:

* `RemoveUnneededTimes` no longer works with stop time updates that are
  missing a `stop_sequence`. This should not have happened anyway since
  ac72186, which formalized the requirement that all stop time updates
  should have a stop sequence for merging to work correctly.

* `RemoveUnneededTimes` and `IncludeStopID` no longer work with GTFS
  feeds that have _only_ a `stop_times.txt`. Like `ScheduledStopTimes`,
  they now require the full chain of `stop_times.txt`, `trips.txt`,
  `routes.txt`, and `agencies.txt` to be present and valid, though the
  files are unused in these filters. Both the old and new behaviors are
  accidents of the implementation details, and the new behaviour could
  be changed if we wanted to explicitly support invalid "partial" GTFS
  feeds, but currently we have no need for this.

…ture