Check directory for latest number when returning configuration

[tpoliaw]

The current scan mutation used to get the next scan number checks both the DB and the fallback directory (GDA var) to find the highest number. When querying the currently highest number (via the configuration admin query), only the DB is checked. This should return the highest number of the the two (directory and DB) but not increment either of them.

configuration(beamline: "ixx") {
    latestScanNumber
}

followed by

scan(beamline: "ixx", visit: "...") {
    scanNumber
}

should always return consecutive numbers (assuming no other requests made in between).