AthenaCommon: Right-align event/slot number in threaded jobs

Change the MessageSvc format such that event and slot number are right
aligned. For high event numbers this avoids that the two fields are
running into each other and is in general more visually appealing.
Also increase the field width of the message level from 7 to 8
(len('WARNING')=7) to ensure there is at least one space between slot
and message level.

Before:
```
ToolSvc.PublicHello                     14  14     INFO my message to the world: A Public Message!
AthenaHiveEventLoopMgr                  1   1      INFO   ===>>>  done processing event #1, run #0 on slot 1,  1 events processed so far <<<===

ToolSvc.PublicHello                     99989      INFO my message to the world: A Public Message!
AthenaHiveEventLoopMgr                  1...11     INFO   ===>>>  start processing event #10000, run #0 on slot 11,  9986 events processed so far  <<<===
```

After:
```
ToolSvc.PublicHello                       14  14   INFO my message to the world: A Public Message!
AthenaHiveEventLoopMgr                     1   1   INFO   ===>>>  done processing event #1, run #0 on slot 1,  1 events processed so far <<<===

ToolSvc.PublicHello                     1...   7   INFO my message to the world: A Public Message!
AthenaHiveEventLoopMgr                  9994   4   INFO   ===>>>  done processing event #9994, run #0 on slot 4,  9995 events processed so far <<<===
```