IMPALA-1003: Speed up compute stats by avoiding null counting for all types and max/avg for fixed-size types.

Removing the Null counting as well as the MAX and AVG calculations
for the types of known size

Change-Id: If218b41fcde0b18218732675585d69342f34e544
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2629
Reviewed-by: Ippokratis Pandis <ipandis@cloudera.com>
Tested-by: jenkins