no more interleaving of tracing log statements

Actual tracing code outputs the tracing informations  with several call to the logger. Those calls are not atomic so several queries being traced at once may become interleaved.

With this PR, the tracing output would change slightly, e.g. with logs generated thanks to info.archinnov.achilles.internal.statement.wrapper.RegularStatementWrapperTest#should_trace_query, we get :
```

TRACE [18:32:27,308][] info.archinnov.achilles.test.sample.entity.Entity1@:tracing Query tracing at host Mock for Host, hashCode: 56410161 with achieved consistency level ALL 
****************************
Description                                                                      | Source           | Source elapsed in micros | Thread name         
description                                                                      | olivier/127.0.1.1 | 100                      | thread              
****************************
```

instead of 

```
TRACE [18:31:34,978][] info.archinnov.achilles.test.sample.entity.Entity1@:tracing Query tracing at host Mock for Host, hashCode: 1194160590 with achieved consistency level ALL  
TRACE [18:31:35,004][] info.archinnov.achilles.test.sample.entity.Entity1@:tracing **************************** 
TRACE [18:31:35,005][] info.archinnov.achilles.test.sample.entity.Entity1@:tracing Description                                                                      | Source           | Source elapsed in micros | Thread name          
TRACE [18:31:35,011][] info.archinnov.achilles.test.sample.entity.Entity1@:tracing description                                                                      | olivier/127.0.1.1 | 100                      | thread               
TRACE [18:31:35,012][] info.archinnov.achilles.test.sample.entity.Entity1@:tracing **************************** 
```