This class computes the generation id used for a replication domain.
It is a checksum based on some special entries/attributes of the domain.
The written stream to this class is the LDIF representation of the entries
we are interested in for computing the generation id. The current
implementation simply does the sum of each written byte and stores the value
in a long. We do not care about the cycling long as the probability of 2
data sets having the same checksum is very low.