ENGINX ARACCESS: Read from block rather than instrument archive

[DominicOram]

As a user I would like the data that I'm getting from ARACCESS to be reliable. As of #5789 the instrument archive is no longer considered reliable, but the block archive is.

Acceptance Criteria

• ARACCESS looks at the block archive rather than the instrument archive

[John-Holt-Tessella]

Check the code but from memory I think that what this means is that the block archiver needs its config file to include all PVs which currently have anything to do with logging. Then these values are stored in the block archive; ARACCESS only looks at the db underneath which is shared byu both archivers.