[UWS] Fix recurrent ConcurrentModificationException during service overload.

This error occurred generally during the backup process while trying to
backup the job list of a specific user. If several of his jobs were running
and changing state during the backup process, this
ConcurrentModificationException was thrown. This generally happens when the same
user submits a lot of shorts jobs in the same time.

This exception was due to a non thread-safe usage of
UWSParameters.additionalParams. To fix this issue, instead of creating it as a
normal HashMap, it is now created as a ConcurrentHashMap.

The same modification has also been applied to UWSParameters.params. In addition
of the replacement of HashMap into ConcurrentHashMap, all `synchronized` blocks
have been removed....there should not be needed any more.