Remove CreditManager from Http2Stream (#32624)

Remove CreditManager from Http2Stream (#32624)

The CreditManager implementation supports multiple awaiters, but Http2Stream's CreditManager never has more than one waiter at a time.  We can instead just encode similar logic into Http2Stream, and make its waiter a reusable singleton, such that if we have to allocate it, we can just keep reusing it for all subsequent waits.  This means we avoid the CreditManager allocation per Http2Stream as well as the Waiter allocation per wait (other than the first).