[concurrency] Reworking the GIL

[Antoine Pitrou]

> As the message on python-dev says: make switching latencies more
> predictable (rather than wildly varying), and reduce the overhead of the
> GIL itself (in terms of spurious system calls, for example, and
> therefore of wasted CPU time with multiple threads).

Ah, the third chief weapon is to replace the obscure opcode-based
setting (sys.setcheckinterval()) with a time-based setting:
sys.setswitchinterval(seconds), the default being 5 ms.