slurmdbd - become SlurmUser before initing plugins

This matches what slurmctld does.

In a particular case, JWT key is read as root by slurmdbd whereas as
SlurmUser by slurmctld. Specifically, any file read by slurmdbd before
becoming SlurmUser is read as root. This collides with NSF filesystems,
where it is common to disable root access, thus making JWT key file read
fail.

Bug 14952