[scheduler] Move generic Scheduler DB methods out of OStoreDB

Description

Fixes to currently broken CTA_PGSCHED flag compilation of main branch:

• moved the methods below to SchedulerDatabase base class since they are Scheduler DB type invariant
• removed OStoreFixure class existing previously in the tests to SchedulerDatabaseFixture
• fixed issues in GenericSchedulerTest.cpp (compiled only for CTA_PGSCHED for now)

testReserveDiskSpace
reserveDiskSpace

getArchiveMountPolicyMaxPriorityMinAge
getHighestPriorityArchiveMountPolicyName
getLowestRequestAgeArchiveMountPolicyName
getRetrieveMountPolicyMaxPriorityMinAge
getHighestPriorityRetrieveMountPolicyName
getLowestRequestAgeRetrieveMountPolicyName

Checklist

• Documentation reflects the changes made.
• Merge Request title is clear, concise, and suitable as a changelog entry. See this link

References

Closes #1190 (closed)