imap-hibernate: Preserve mail_user.session_create_time through hibernation

imap-hibernate: Properly fix hibernation The previous fix did not properly fix hibernation as the clients still dropped out of hibernate. Also the tag used was no longer following. This change will track tag changes and keeps the hibernation process going on until the user enters something else than DONE\r\ntag IDLE\r\n in same packet.

imap-hibernate: If imap-master socket is busy, retry un-hibernation later.

imap-hibernate: send connect/disconnect events to anvil make doveadm who + mail_max_userip_connections work with hibernated connections.

imap-hibernate: Use sockets' st_dev and st_ino to verify that the passed fd is correct.

Changed type of internet port values to in_port_t everywhere. Created special SET_IN_PORT setting type for internet port values. Created net_str2port() for parsing internet port values. Removed several atoi() invocations in the process.

Added imap-hibernate process for gathering IDLEing imap processes. imap_hibernate_timeout setting controls how quickly the connection is moved from imap process to imap-hibernate process. Some IMAP extensions like NOTIFY, SEARCH=CONTEXT and COMPRESS aren't supported yet. There's also a new X-STATE command, which can be used to export the current IMAP connection state to a string and later on imported to get back to the original state (a quick resync feature for IMAP clients). However, this command is disabled for now due to the current code being unoptimized for untrusted input.