Solve some nasty deschedulinging races with a lock

Right now, as pointed out in #8132, it is very easy to introduce a subtle race
in the runtime. I believe that this is the cause of the current flakiness on the
bots.

I have taken the last idea mentioned in that issue which is to use a lock around
descheduling and context switching in order to solve this race.

Closes #8132
auto merge of #10817 : alexcrichton/rust/sched-fix, r=brson

Right now, as pointed out in #8132, it is very easy to introduce a subtle race
in the runtime. I believe that this is the cause of the current flakiness on the
bots.

I have taken the last idea mentioned in that issue which is to use a lock around
descheduling and context switching in order to solve this race.

Closes #8132