concurrency: Remove acquireExclusiveLock

During getUser, select from OUser reads
It does not need an exclusive lock

This logic is similar to @lvca commit
Minor: removed lock on authentication at
https://github.com/orientechnologies/orientdb/commit/000e40420feb842f4e1182f2c08b6e3010547a22

Reviewed-by: Lloyd Chang <lloydchang@gmail.com>
Signed-off-by: Lloyd Chang <lloydchang@gmail.com>