[dovecot/core] 3dcf98: Add <%{pid}> to default mail_log_prefix

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/dovecot/core
  Commit: 3dcf98a55df71b11c2cac4c20232b07e49606a9a
      https://github.com/dovecot/core/commit/3dcf98a55df71b11c2cac4c20232b07e49606a9a
  Author: Timo Sirainen <timo.sirainen at dovecot.fi>
  Date:   2017-09-21 (Thu, 21 Sep 2017)

  Changed paths:
    M doc/example-config/conf.d/10-logging.conf
    M src/lib-storage/mail-storage-settings.c

  Log Message:
  -----------
  Add <%{pid}> to default mail_log_prefix

It's especially useful nowadays when the same session ID can be reused by
multiple processes with IMAP hibernation enabled.