Fix a race at thread creation

At thread creation start_routine() is called
and from there the virtual function idle_loop()
because we do this inside Thread c'tor, where the
virtual mechanism is disabled, it could happen that
the base class idle_loop() is called instead.

The issue happens with TimerThread and MainThread
where, at launch, start_routine calls
Thread::idle_loop instead of the derived ones.

Normally this bug is hidden because c'tor finishes
before start_routine() is actually called in the
just created execution thread, but on some platforms
and in some cases this is not guaranteed and the
engine hangs.

Reported by Ted Wong on talkchess

No functional change.
Rework Thread hierarchy

Introduce ThreadBase struct that is search
agnostic and just handles low level stuff,
and derive all the other specialized classes
form here.

In particular TimerThread does not hinerits
anymore all the search related stuff from Thread.

Also some renaming while there.

Suggested by Steven Edwards

No functional change.