Polyswarm-client

* **Feature** - Push metadata to IPFS if it matches the Verdict Schema in polyswarm-artifact (Not breaking)
* **Feature** - Add Ambassador bounty submission env vars to dynamically set rate of bounty submissions
* **Fix** - Pass artifact type on second on_bounty_post_failed call
* **Fix** - Remove stop and join when scanner setup fails

* **Feature** - Add `setup()` to `AbstractScanner` for setting up AV services in a worker

* **Fix** - Increment counter on scan results for use in horizontal scaling of workers
* **Fix** - Reset failed tries counter on successful worker response

* **Fix** - Cap websocket retry connection backoff wait time
* **Feature** - Add artifact type handling & filtering

Breaking Changes
1. `AbstractAmbassador.push_bounty()` parameters changed to `(self, artifact_type, amount, ipfs_uri, duration, chain, api_key)`
1. `AbstractAmbassador.on_bounty_post_failed()` parameters changed to `(self, artifact_type, amount, ipfs_uri, expiration, chain)`
1. `AbstractAmbassador.on_before_bounty_posted()` parameters changed to `(self, artifact_type, amount, ipfs_uri, expiration, chain)`
1. `AbstractAmbassador.on_after_bounty_posted()` parameters changed to `(self, guid, artifact_type, amount, ipfs_uri, expiration, chain)`
1. `AbstractMicroengine` and `AbstractArbiter` `.fetch_and_scan_all()` parameters changed to `(self, guid, artifact_type, uri, vote_round_end, chain)`
1. `AbstractMicroengine` and `AbstractArbiter` `.scan()` parameters changed to `(self, guid, artifact_type, content, chain)`
1. `AbstractScanner.scan()` parameters changed to `(self, guid, artifact_type, content, chain)`
1. `PolyswarmClient.on_new_bounty` event parameters changed to `(self, guid, artifact_type, author, amount, uri, expiration, block_number, txhash, chain)`
1. `Producer.scan()` parameters changed to ` (guid, artifact_type, uri, expiration_blocks, chain)`

* **Fix** - Handle ServerDisconnectError

* **Feature** - Add arbiter producer backend
* **Fix** - Reduce CancelledError log severity