`BlockReason` in `threadStatus` is inaccurate

[MaximilianAlgehed]

BlockReason in threadStatus is BlockedOnOther too often. This needs fixing by e.g. taking MVars and STM transactions into account.

[MaximilianAlgehed]

From @coot:

I see one problem with this implementation: it does not work well with our implementation of MVar's which is based on TVar's. When a thread is blocked on an MVar we will thus report ThreadBlocked BlockedOnSTM. But it shouldn't be difficult to distinguished MVars and TVars. We's need to add a boolean flag to TVar: tvarIsMVar :: Bool. The problem to solve is how to update it when we create an MVar in newEmptyMVarDefault. The simplest way would be to move that function to io-sim, where we have access to TVar internals.