Per-thread Haskell thread list/numbering (remove global lock from thread allocation)

Currently, we take out the scheduler lock when a thread is allocated, in order to make sure we allocate a fresh thread ID and can attach it to the global thread list in the nursery. If we partition the thread ID space per originating capability and maintain thread lists per OS thread, we can remove this lock.

Marking low priority because no one seems to be running into this as their particular scaling problem (and there may be other reasons why high thread churn doesn't scale across cores).

Trac metadata

Trac field	Value
Version	7.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Runtime System
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonmar
Operating system
Architecture

changed weight to 3

added RTS Tbug Trac import labels

added runtime perf label

added Plow label