sleep mode

Sleep mode is defined that the mariadbd mode should not use any cpu.
This will help for people having mariadbd running on their desktop or mariadbd running at a dbaas provider without anything to do.

Sleep mode should start when there is no users connected, no events are running and there is nothing on the replication stream for a some time (a minute?).

• Background cleanup activities and checkpoints can continue to their end before going
  to sleep.
• During sleep mode:
• All worker threads should go to sleep
• Checkpoints
• InnoDB cleanup threads
• Push notifications
• Semi sync and heartbeats
• Potentially write changed pages to disk (faster recovery later if
  needed)
• "Ping" (server is alive) should still work.
• One could implement 'ping' by just starting a MariaDB handchake and check that the
  server answers.
• Slaves still have to answer to heart beats during sleep mode.

Wakeup from sleep mode happens if a new user successfully connects, an event starts, there is a replication event received or the server gets a kill request.

Implementation could be done by having a sleep_mode_active variable that is set if the 'sleep_mode' is set and the last user (not counting replication threads) disconnects and there has not been any events from the master for some time (not counting heart beats).

When a background thread (like finished a checkpoint) it should call mariadb_sleep() which will set the slave to sleep if the sleep_mode_active is set.
When a new user connects, an event start or the io thread gets an event, the should call 'mariadb_awake()'. mariadb_awake should just cost a single 'if' if sleep mode is not active.