DBZ-2975: Prototype multiple offset contexts per task

[morozov]

Closes #12. The corresponding section of the design doc.

Change summary:

1. OffsetContext.Loader is parametrized with <O extends OffsetContext> (see DBZ-2975: Extract offset context from object states to method signatures #13 for the reasoning).
2. All messages produced by the SQL Server connector instead of having "server=serverName" as their source partition now have "server=serverName;database=databaseName" as their source partition. This is a breaking change from the PoV of existing users. An existing connector upgraded to this version won't be able to recover from the state produced by the previous version.

New APIs:

1. TaskPartition interface. Describes the partition of the source to be processed by the task in terms of a given connector and provides its representation as a Kafka Connect source partition. For instance, an SqlServerTaskPartition describes a source database. Any other connector can implement this interface in its specific terms.
2. TaskPartition.Provider interface. Provides a collection of task-specific partitions for a given task based on its configuration. E.g. a list SqlServerTaskPartitions each representing a database for a SQL Server connector task.
3. TaskOffsetContext class. It's effectively a map of TaskPartition to OffsetContext describing the scope of work for a given task and its state. This map will be iterated over by different task components to process multiple databases.
4. TaskOffsetContext.Loader class. Similarly to OffsetContext.Loader, loads the task offset context.