Add existence check on pusher channels for logs

First stab at checking when we need to send pusher messages with logs and when we can ignore them

More in the commit message:

```
We plan to stop sending log updates to log channels which are not
occupied, because logs produce a quite high volume of messages. This
commit is first step in that direction, it adds:

* a simple web endpoint which can handle pusher webhooks and register
  a channel as either occupied or vacant
* metrics measuring how many log parts would we need to send and how
  many we can simply ignore
```
may the `log_` in `log_channels_existence_check` and `log_channels_existence_metrics` could be removed since this is travis-logs specific