Sink: Support writing to multiple buckets

Suggested Release Note:

A single instance of the Sink connector can now write to different buckets depending on which topic a record was received from. Sink config properties that accept a qualified collection name (like `myScope.myCollection`) now accept an optional bucket prefix, like `myBucket.myScope.myCollection.` If the bucket name component is not specified, it defaults to the value of the `couchbase.bucket` config property.

The sink config properties that now accept the bucket name prefix are `couchbase.default.collection` and `couchbase.topic.to.collection.map`. For these properties, a bucket name component that contains a dot (`.`) must be escaped by enclosing the name in backticks, since a dot is both the component delimiter and a valid character in a bucket name.