Rename `TrackingBatch` batch-specific fields to match Transact protocol Batch

[shannynalayna]

Currently, Grid's TrackingBatch has batch_header and serialized_batch fields. The batch_header field is a String, populated by a Transact batch's header_signature. Grid's tracking batch serialized_batch field is taken from the transact batch's batch_header field. This naming is confusing as the batch_header field in the Grid struct is pulling the header_signature field from Transact, yet the Transact batch_header field is being used in the serialized_batch field in Grid. The batch_header field should be renamed to better reflect what this field is.