Check the length of aggregate metadata

PostgreSQL gives an error if you try to shove 256+ characters
into a 255-character field. So we need raise a HTTPBadRequest
explanation if the length of aggregate_metadata.keys or
aggregate_metadata.values is above 255.

Change-Id: I2c778f2237ba5bd2aa8335a0eae80f3aad3e9157
Closes-Bug: #1219693
Merge "Check the length of aggregate metadata"