IMAP: simplify locking and fix potential lock loss

Run the locked code under 'with': this guarantees that lock
will be released in any case.

This modification also avoids the case when our thread wasn't running
locked when exception was caught, another thread got the lock, our
code checked it via self.connectionlock.locked() and errorneously
released the lock thinking that is was running locked.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap/imapserver.py

@@ -297,10 +297,9 @@ class IMAPServer:
                 self.delim = imaputil.dequote(self.delim)
                 self.root = imaputil.dequote(self.root)
 
-            self.connectionlock.acquire()
+            with self.connectionlock:
                 self.assignedconnections.append(imapobj)
                 self.lastowner[imapobj] = curThread.ident
-            self.connectionlock.release()
             return imapobj
         except Exception as e:
             """If we are here then we did not succeed in getting a
@@ -308,9 +307,6 @@ class IMAPServer:
             error..."""
             self.semaphore.release()
 
-            if(self.connectionlock.locked()):
-                self.connectionlock.release()
-
             severity = OfflineImapError.ERROR.REPO
             if type(e) == gaierror:
                 #DNS related errors. Abort Repo sync