auth: add locking to `PbsAuthenticator` to avoid race conditions

auth: add locking to `PbsAuthenticator` to avoid race conditions

currently we don't lock the shadow file when removing or storing a
password. by adding locking here we avoid a situation where storing
and/or removing a password concurrently could lead to a race
condition. in this scenario it is possible that a password isn't
persisted or a password isn't removed. we already do this for
the "token.shadow" file, so just use the same mechanism here.

Signed-off-by: Shannon Sterz <s.sterz@proxmox.com>