send-email: --batch-size to work around some SMTP server limit

Some email servers (e.g. smtp.163.com) limit the number emails to be
sent per session (connection) and this will lead to a faliure when
sending many messages.

Teach send-email to disconnect after sending a number of messages
(configurable via the --batch-size=<num> option), wait for a few
seconds (configurable via the --relogin-delay=<seconds> option) and
reconnect, to work around such a limit.

Also add two configuration variables to give these options the default.

Note:

  We will use this as a band-aid for now, but in the longer term, we
  should look at and react to the SMTP error code from the server;
  Xianqiang reports that 450 and 451 are returned by problematic
  servers.

  cf. https://public-inbox.org/git/7993e188.d18d.15c3560bcaf.Coremail.zxq_yx_007@163.com/

Signed-off-by: xiaoqiang zhao <zxq_yx_007@163.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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