Change normalised duration fields to BIGINT

Change normalised duration fields in server schema to BIGINT to avoid
integer overflow.
Remove use of ServiceLevel in server-extra.sql

ServiceLevel is not stored in NormalisedSuperSummaries so either the
normalised values or a proxy for ServiceLevel is used.

- Remove ServiceLevel and division by 1000 from VSumCPU and VUserCPU views
  as normalised durations are used.
- Replace ServiceLevel in VSpecIntHistory and VHepSpecHistory views with
  calculation based on NormalisedWallDuration and WallDuration as