kvflowcontrol: add store stream token watcher interface

[kvoli]

Define a new interface in kvflowcontrol which supports a similar interface to:

// TokenWaitingHandle is the interface for waiting for positive tokens.
type TokenWaitingHandle interface {
	// WaitChannel is the channel that will be signaled if tokens are possibly
	// available. If signaled, the caller must call
	// TryDeductAndUnblockNextWaiter.
	WaitChannel() <-chan struct{}
	// TryDeductAndUnblockNextWaiter is called to deduct some tokens. The tokens
	// parameter can be zero, when the waiter is only waiting for positive
	// tokens (such as when waiting before eval). granted <= tokens and the
	// tokens that have been deducted. haveTokens is true iff there are tokens
	// available after this grant. When the tokens parameter is zero, granted
	// will be zero, and haveTokens represents whether there were positive
	// tokens. If the caller is unsatisfied with the return values, it can
	// resume waiting using WaitChannel.
	TryDeductAndUnblockNextWaiter(tokens kvflowcontrol.Tokens) (granted kvflowcontrol.Tokens, haveTokens bool)
}

// StoreStreamSendTokensWatcher implements a watcher interface that will use
// at most one goroutine per SendTokenCounter and WorkClass that has no send
// tokens. Replicas (from different ranges) waiting for those tokens will call
// NotifyWhenAvailable to queue up for those send tokens.
type StoreStreamSendTokensWatcher interface {
	// NotifyWhenAvailable queues up for tokens for a given token counter and
	// work class. When tokens are available, tokensGrantedNotification is
	// called. It is the caller's responsibility to call CancelHandle() when
	// tokens are no longer needed, or when the caller is done.
	NotifyWhenAvailable(
		stc TokenCounter,
		wc admissionpb.WorkClass,
		tokensGrantedNotification TokenAvailableNotification,
	) StoreStreamSendTokenHandleID
	// UpdateHandle updates the given handle to watch the given work class,
	// removing it from watching the existing work class, if the work class is
	// different.
	UpdateHandle(handle StoreStreamSendTokenHandleID, wc admissionpb.WorkClass)
	// CancelHandle cancels the given handle, stopping it from being notified
	// when tokens are available.
	CancelHandle(handle StoreStreamSendTokenHandleID)
	// String returns a string representation of the store stream send tokens
	// watcher.
	String() string
}

const InvalidStoreStreamSendTokenHandleID StoreStreamSendTokenHandleID = 0

// TokenAvailableNotification is an interface that is called when tokens are
// available.
type TokenAvailableNotification interface {
	// Notify is called when tokens are available to be granted.
	Notify(context.Context)
}

See https://github.com/sumeerbhola/cockroach/blob/2064b279e95bbd68b0a7898503168d384cd0158c/pkg/kv/kvserver/kvflowcontrol/kvflowconnectedstream/store_stream.go#L142-L192

Jira issue: CRDB-40750

Epic CRDB-37515