IDs are not re-issued

Committed on behalf of Jon Olav Hauglid <jon.hauglid@oracle.com>.

With this patch the currently used thread IDs are maintained in a sorted
list so that it can be checked that each new connection is assigned an ID
which is not currently in use. This prevents reuse of active IDs in cases
where the 32 bit counter wraps around. The counter is kept at 32 bit as it
is part of the client protocol.

The patch makes code more consistent by using the my_pthread_id typedef
instead of various uint/ulong variants.

The patch also changes the thread_created counter to be incremented using
atomics rather than protected by a separate mutex. The list of active THDs
is changed from map to a sorted array.

Combined this gives no reduced connect/disconnect performance even with the
added processing required to assign unique IDs.