Allow Bitbases::init() to be called more than once

Allow Bitbases::init() to be called more than once

Currently if we call it more than once, we crash.

This is not a real problem, because this function is
indeed called just once. Nevertheless with this small fix,
that gets rid of a hidden 'static' variable, we cleanly
resolve the issue.

While there, fix also ThreadPool::exit to return in a
consistent state. Now all the init() functions but
UCI::init() are reentrant and can be called multiple
times.

No functional change.