Adding a background process executor, and observer.

We will be using the external utilities like pg_dump, pg_dumpall,
pg_restore in background. pgAdmin 4 can be run as a CGI script, hence -
it is not good idea to run those utility in a controlled environment.
The process executor will run them in background, and we will execute
the process executor in detached mode.

Now that - the process executor runs in detached mode, we need an
observer, which will look at the status of the processes. It also reads
output, and error logs on demand.

Thanks - Surinder for helping in some of the UI changes.

web/pgadmin/model/__init__.py

@@ -168,3 +168,22 @@ class DebuggerFunctionArguments(db.Model):
                 nullable=False)
 
     value = db.Column(db.String(), nullable=True)
+
+
+class Process(db.Model):
+    """Define the Process table."""
+    __tablename__ = 'process'
+    pid = db.Column(db.String(), nullable=False, primary_key=True)
+    user_id = db.Column(
+            db.Integer,
+            db.ForeignKey('user.id'),
+            nullable=False
+            )
+    command = db.Column(db.String(), nullable=False)
+    desc = db.Column(db.String(), nullable=False)
+    arguments = db.Column(db.String(), nullable=True)
+    logdir = db.Column(db.String(), nullable=True)
+    start_time = db.Column(db.String(), nullable=True)
+    end_time = db.Column(db.String(), nullable=True)
+    exit_code = db.Column(db.Integer(), nullable=True)
+    acknowledge = db.Column(db.String(), nullable=True)