Added more docblocks for interfaces + removed unused uses

src/PostgresDriver/ArrayCursorDriver.php

src/PostgresDriver/Cursor.php

src/PostgresDriver/CursorPosition.php

src/PostgresDriver/ICursor.php

src/PostgresDriver/ICursorDriver.php

src/PostgresDriver/ICursorFactory.php

src/PostgresDriver/PostgresCursorDriver.php

src/PostgresDriver/PostgresCursorFactory.php

src/PostgresDriver/SemanticCursor.php

src/PostgresDriver/TrackedCursor.php

tests/Store/PostgresDriver/CursorTest.phpt

tests/Store/PostgresDriver/PostgresCursorTest.phpt

tests/Store/PostgresDriver/SemanticCursorIntegration.phpt

tests/Store/PostgresDriver/TrackedCursorTest.phpt