Add data type for abstract statistics
Add data type for abstract statistics

Our cluster score is a weighted sum of certain sums and
standard deviations of node characteristics. When placing
a single instance, the cluster score of a big number of
quite similar clusters are computed: that of the original
cluster with one instance added, for each possible placement.
To speed up these computations, add an abstract type of
statistics that allows a potentially more efficient update
of a single value.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Petr Pudlak <9d19625f8ee825a2c398462ed335cbcf98f79c32@google.com>

dimara/ganeti,leshchevds/ganeti,ganeti/ganeti,ganeti-github-testing/ganeti-test-1,grnet/snf-ganeti,grnet/snf-ganeti,dimara/ganeti,onponomarev/ganeti,mbakke/ganeti,apyrgio/snf-ganeti,yiannist/ganeti,andir/ganeti,leshchevds/ganeti,andir/ganeti,ganeti/ganeti,andir/ganeti,apyrgio/ganeti,yiannist/ganeti,yiannist/ganeti,bitemyapp/ganeti,ganeti-github-testing/ganeti-test-1,mbakke/ganeti,onponomarev/ganeti,leshchevds/ganeti,ganeti/ganeti,apyrgio/ganeti,bitemyapp/ganeti,apyrgio/snf-ganeti,mbakke/ganeti