* Make all UvRequest objects normal GC handles
- This avoids the cost of using GCHandle.Alloc per operation.
- It *does* mean that we need to explicitly dispose UvRequest objects
after using them (which we did before anyways). This change does
add a few try catch statements to make sure we always dispose the UvRequest
if there are synchronous exceptions.
- This is ~1.5% of the overhead in the benchmarks today
- Keep track of all allocated UvRequest objects with a WeakReference in DEBUG
and assert none are kept around after cleaning up.
- Fixed a leak where we don't clean up UvWriteReq objects when writing
to the named pipe.