wrap range-spanning ops in txn

added a new OpRequiresTxn error which can be returned if an operation
requires to be carried out in a Txn but is not.
Upon encountering such an error, the txn_coord_sender will re-send
the operation within a transaction.

The way in which this is done currently feels extremely clumsy and
should be improved.

implemented the proto.Combinable interface for InternalResolveIntentResponse,
which can be addressed to ranges.