Adds ability to remove annotation keys

[looselycoupled]

This is for annotations only. Adds the ability to completely replace annotations.

Behind the scenes, this is done by pulling the most recent annotations, comparing keys, and providing a list of keys that should be removed (as this is how the protobuf call is structured).

Tests have been updated as well as the documentation.

[bbengfort]

Looks good - my comments were about making the documentation more clear to users. I always forget how this works, so I'd certainly appreciate some guideposts in the documentation!

As we just discussed over my monitor, this change does not affect tags, so we should be much clearer in the documentation that only annotations are affected by upsert. In particular, we could note that tags cannot be deleted, only set to empty string (we should check if this is true) and that if a tag is set that is not allowed a btrdb.exceptions.BTrDBError: [408] (408: tag key "foo" is invalid) is raised.

(If tags can be deleted, though, it might just be easier to make them behave the same way).

[bbengfort]

I think that a bit more documentation might be helpful here, along with an example. I keep getting confused and forgetting what's happening with update and have to try it on husky before I do it in real life. I'm thinking something to the effect of:

Updates metadata including tags, annotations, and collection as an UPSERT operation. 

By default, the update will only affect the keys and values in the specified tags and annotations 
dictionaries, inserting them if they don't exist, or updating the value for the key if it does exist. If 
any of the update arguments are None, they will remain unchanged in the database. 

To delete either tags or annotations, you must specify exactly which keys and values you want
set for the field and set `replace=True`. For example:

    >>> annotations, _ = stream.anotations()
    >>> del annotations["key_to_delete"]
    >>> stream.update(annotations=annotations, replace=True)

This ensures that all of the keys and values for the annotations are preserved except for the
key to be deleted. 

Parameters
-----------
tags : dict, optional
    Specify the tag key/value pairs as a dictionary to upsert or replace. If None, the tags 
    will remain unchanged in the database. 
annotations : dict, optional
    Specify the annotations key/value pairs as a dictionary to upsert or replace. If None,
    the annotations will remain unchanged in the database. 
collection : str, optional
    Specify a new collection for the stream. If None, the collection will remain unchanged.
encoder : json.JSONEncoder or None 
    JSON encoder class to use for annotation serialization. Set to None to prevent JSON
    encoding of the annotations.
replace : bool, default: False
    Replace all annotations or tags with the specified dictionaries instead of performing 
    the normal upsert operation. Specifying True is the only way to remove annotation keys.

[bbengfort]

Could we also add the example from above about how to delete annotations?

[looselycoupled]

Yup, I can add this after I'm done with current tasking and will then merge this PR

[immesys]

LGTM