WL#1369: Allow setting temporary directory to be set online and per thread

Affects: Server-7.1   —   Status: Un-Assigned   —   Priority: Medium

Rather typical problem of our customers is running out of space in "tmpdir"
running some large queries or ALTER TABLE queries. It is especially common with
customers having "tmpdir" pointing to swapfs which is limited in side.

So we shall do the following

1) Allow setting "tmpdir" without restarting the server.  
2) Allow setting "tmpdir" on session basics.

Note this task is linked to WL#1243  (Separate tmpdir for slave) which shall be
trivial to do as soon as this task is done.