The Great Rename

Since we started KtMongo, some naming and structural decisions have been discovered to be suboptimal. This MR reorganizes all of them.

After this is merged, no future major changes are planned.

• ….dsl.expr → ….dsl.query, because "expression" is tied to aggregations in the official documentation.
• FilterOperators, UpdateOperators, etc → FilterQuery, UpdateQuery because they are collectively called "query operators" in the official documentation.
• FilterExpression, UpdateExpression have been removed from the public API and replaced by factory methods, to reduce API surface.
• ….dsl.models → ….dsl.command, because that's how the official documentation refers to them.
• Expression (and its hierarchy) has been renamed to BsonNode and moved from ….dsl.expr.common to ….dsl.tree closer to Node, to make their relationship more explicit and more a package.
• ….dsl.options have been moved to their respective command class.
• ….dsl.options.common have been moved to ….dsl.options to remove a package.
• ValueDsl → AggregationOperators. I was considering naming it something that contains "Expression" but couldn't find a good name.

Closes #35 (closed)