purge_coordinator_state::do_purge(): Simply use all innodb_purge_threads,
no matter what the LSN age is. During shutdown with innodb_fast_shutdown=0
this code could degrade to using only 1 thread.

Also, restore periodical "InnoDB: to purge" messages that were
accidentally disabled in commit 80585c9d.

Reviewed by: Vladislav Lesin and Vladislav Vaintroub