After the improvements made in WL#6606, performance testing has
shown that the major bottleneck for connect/disconnect performance
now is the LOCK_thd_count mutex.

This patch increases the connect/disconnect performance by
splitting LOCK_thd_count so that it no longer protects
several different structures/variables. LOCK_thd_count
is now only used to protect the list of connections (THDs).

Changes made to existing usage of LOCK_thd_count:

LOCK_thread_cache is introduced to protect the thread cache used
by the default connection handler (one thread per connection).

Synchronization during startup of signal handler thread is now
done using LOCK_start_signal_handler.

Synchronization during shutdown of main thread connection
listening is now done using LOCK_socket_listener_active.

The global thread_id counter is now incremented using atomics,
rather than being protected by LOCK_thd_count.

THD::current_linfo is now protected by THD::LOCK_thd_data rather
than LOCK_thd_count.

max_used_connections is now reset under protection of
LOCK_connection_count rather than LOCK_thd_count.