Implement key/value quota-overrun detection in LocalTransport

We now preflight the effect of a k/v backup delta set against the
current contents of the datastore, so we can detect and report a
quota overrun nondestructively.

Test: bit CtsBackupTestCases:.KeyValueQuotaTest
Change-Id: If69687b891271d7bbb46c3a66728b716dc51fc39