DBZ-2975: Extract offset context from object states to method signatures

[morozov]

Closes #8.

It should be possible to reuse the graph of components under ChangeEventSourceCoordinator for processing multiple partitions represented via OffsetContext sequentially. Specifically, StreamingChangeEventSource#execute() and SnapshotChangeEventSource#execute() should accept an offset context and explicitly pass it down the line.

The following classes and hierarchies no longer have the offset context as part of their state which is now part of their interface:

1. ChangeEventSourceFactory (including all connector implementations).
2. SnapshotChangeEventSource (including snapshot/streaming interfaces and all connector implementations).
3. Connector-specific classes (mysql.EventBuffer).

Certain classes are parametrized with <O extends OffsetContext>. Since the graph of components under the SourceTask implementation will be dealing with a set of contexts, there may be no place to do typecasting for the entire set. Additionally, the use of parameterized types looks safer:

1. BaseSourceTask
2. ChangeEventSourceCoordinator
3. ChangeEventSourceFactory
4. SnapshotChangeEventSource
5. SnapshotResult