Fix: The email notification of review comments gets stuck.

Sometimes it is found that one thread goes stuck when waiting
for an answer from the SMTP server.

Fixed. Enable user to

 -config the timeout value of opening a socket connected to a remote
  SMTP server.

 -config the worker-thread pool size of executor used for sending out
  review comments notification when it is not enough to dedicate only
  one thread.

 -config the default size of the background execution thread pool when
  one thread is not enough to handle miscellaneous tasks including
  sending out every kind email notification.

Change-Id: Id8177b374f7049cfac617c50e66b2c83ae71641b
6 files changed