Wtfix

**Fixes**

- Set logging level of 'wtfix' logger to `LOGGING_LEVEL` when it is first requested.
- Fix bug in `_replay_buffered_messages` that prevented buffered messages from being received.

**Enhancements**

- Fix Python 3.8 compatibility issues.
- Switch to using a context manager for managing the active FIX connection / protocol.

**Enhancements**

- **BREAKING CONFIG CHANGES** (please update your settings files in `wtfix.config.settings`):
- The MESSAGE_STORE parameters now form part of the CONNECTION section. This allows individual message stores to
be configured when multiple connections need to be run simultaneously.
- Add configuration option for specifying which JSON encoder / decoder to use when adding messages to a message
store.
- The FIX protocol and version can now be configured for individual CONNECTIONS. This lays the foundation for
supporting various different protocols and versions in the future. FIX 4.4 is currently the default.

- Add `PipelineTerminationApp` and use `del` to encourage the Python interpreter to garbage collect a message once it
has reached either end of the pipeline.
- Upgrade aioredis dependency to version 1.3
- Remove dependency on [unsync](https://github.com/alex-sherman/unsync) which has become largely redundant with the new async features released as part
of Python 3.7.
- Now requires Python >= 3.7.

**Fixes**

- Update dependencies to latest versions.
- Change `RedisPubSubApp` to allow subclasses to override the Redis sending channel name via
`RedisPubSubApp.SEND_CHANNEL`

**Fixes**

- Fixed encoding of `PossDupFlag` in `EncoderApp`.
- Fixed setting of `OrigSendingTime` tag for messages that are resent.

**Fixes**

- `ClientSessionApp`: handle `NoneType` error when shutting down.