Fix a performance problem in DIMEMessage._bytestream_to_pdv. Its cost was quadratic to the length of the input.
Fix a performance problem in DIMEMessage._bytestream_to_pdv. Its cost was
quadratic to the length of the input.

scaramallion/pynetdicom,scaramallion/pynetdicom,scaramallion/pynetdicom3,scaramallion/pynetdicom3