Added 'RetrievePreprocessor' class skeleton.

Signed-off-by: Cristiano Urban <cristiano.urban@inaf.it>

transfer_service/retrieve_preprocessor.py

+from config import Config
+from db_connector import DbConnector
+from task_executor import TaskExecutor
+
+
+class RetrievePreprocessor(TaskExecutor):
+
+    def __init__(self):
+        config = Config("vos_ts.conf")
+        self.params = config.loadSection("file_catalog")
+        self.dbConn = DbConnector(self.params["user"],
+                                  self.params["password"],
+                                  self.params["host"],
+                                  self.params.getint("port"),
+                                  self.params["db"])
+        super(RetrievePreprocessor, self).__init__()
+
+    def run(self):
+        print("Starting retrieve preprocessor...")
+        self.setSourceQueueName("read_pending")
+        self.setDestinationQueueName("read_ready")
+        while True:
+            self.wait()
+            if self.destQueue.len() < self.maxReadyJobs and self.srcQueue.len() > 0:
+                self.jobObj = self.srcQueue.getJob()                
+                
+                # do something here...
+                
+                self.srcQueue.extractJob()
+                self.destQueue.insertJob(self.jobObj)
+                print(f"Job {self.jobObj.jobId} MOVED from {self.srcQueue.name()} to {self.destQueue.name()}")
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