Change Timer implementation on Unixes to use only one scheduling thread (dotnet/corec...

Change Timer implementation on Unixes to use only one scheduling thread (dotnet/coreclr#7071)

* Change Timer implementation on Unixes to use only one scheduling thread

- Separated from https://github.com/dotnet/corert/pull/7066

* Address feedback from https://github.com/dotnet/corert/pull/7066

* Remove reference to s_lock

* Reduce work inside lock

* Move _id

* Fix duplicate timers in scheduled timer list, move info to TimerQueue

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>
Commit migrated from https://github.com/dotnet/coreclr/commit/6e215e1faa8275c1468bcd059e29a530d2de0064