Snapstream

- Updated dev dependency version constraints to allow major version updates
- Changed _consumer_handler function signature, adding commit_each_message flag
- Moved get_consumer and get_producer into Topic as private methods
- Topic.next uses own Consumer instance that's closed down correctly
- Fixed slicing bug involving start and end slice parameters
- Made underlying Consumer and Producer instances available under Topic.consumer and Topic.producer

- Added locking for rocksdict operations to avoid race conditions
- Calling a handler function manually no longer requires passing a dictionary for the kwargs argument
- Added type hints to curried functions
- Using snapstream cache --stats in the snapstream command will now only show db stats
- Added example docs showing an implementation of API endpoints

- Added locking for rocksdict operations to avoid race conditions
- Calling a handler function manually no longer requires passing a dictionary for the `kwargs` argument
- Added type hints to curried functions

* Fix cli bug that failed getting schema_path from the .snapstreamcfg config file

Not backwards compatible:

* Next to raising kafka message errors when passing raise_error=True, they are raised when error is fatal or non-retriable

Backwards compatible:

* Uses `/etc/secrets` in cli as default path for file secrets

* Uses `secrets_base_path` from the [`.snapstreamcfg`](https://snapstream.readthedocs.io/en/0.0.8/cli.html#cli) file if available

* Uses `schema_path` from the [`.snapstreamcfg`](https://snapstream.readthedocs.io/en/0.0.8/cli.html#cli) file -- for avro deserialization -- if available

* Shows "(catching up)" when messages read by cli tool are created before the command was executed

Not backwards compatible:

- All exceptions (including SystemExit) are now being caught and sent to the main thread, causing not just a thread but the whole program to crash (as it should)

- No longer raises kafka message errors by default, change back to old behavior by passing `raise_error=True` in get_consumer

Backwards compatible:

- AvroCodec now accepts an `avro.schema.Schema` instance besides an .avsc filepath string

- Fixed a bug that prevented reading offset 0 while slicing a topic, because 0 is falsy

- Added slicing, testing, and revamped index docs