Initial tests for Command and Legacy batch classes
Remove MappedBatch::isEmpty() and private documents property

This method is unnecessary since getItemCount() is defined in BatchInterface. The $documents array can also be removed (it was a relic of older methods that allowed fetching a document by an index).
Do not update index map in MappedBatch unless add() is successful
MappedBatch tests
Tests for Bulk API method calls
Rewrite OrderedBulk functional test

Assertions for BulkResult values will be done in a separate test.
Remove obsolete BatchTest