Created a wrapper class for the couple Backend+OStoreDB as the unit tests expect a clean DB each time.

Added function to return the copy No. of RetrieveFromTapeCopyRequest and added the corresponding cpp in the CMakeLists.txt

Simplified the UserIdentity and CreationLog structures in the object store to match the SchedulerDatabase's.

Before this commit the Scheduler class would call

  SchedulerDatabase::queue(const ArchiveToFileRequest &)

in a loop when handling a user request to archive multiple remote files
to a directory in the archive.  This modification opens the door for the
SchedulerDatabase object to remember more about the context of each
archive request.  In this case which archive to directory request was at
the origin of each of the corresponding archive to file requests.

The agent now carries its own timeout (defaulting to 60s).
The unit test now validates that the 2 agents were cleared from the agent register.

ArchiveToFileRequest now includes the archive copy number to tape pool map required by the SchedulerDatabase

Added the unique pointer for sqlite statements with custom deleter and fixed the error reporting for SQLite prepare