osd, common: If agent_work() finds no objs to work on delay 5 (default) secs

Add config osd_agent_delay_time of 5 seconds
Honor delay by ignoring agent_choose_mode() calls
Add tier_delay to logger
Treat restart after delay like we were previously idle

Fixes: #8113
Backport: firefly

Signed-off-by: David Zafman <david.zafman@inktank.com>
osd: Prevent divide by zero in agent_choose_mode()

Fixes: #8175
Backport: firefly

Signed-off-by: David Zafman <david.zafman@inktank.com>
Signed-off-by: Sage Weil <sage@inktank.com>