Table throttler: race condition

[shlomi-noach]

While working on #14164 and #14165 I found a race condition in the tablet throttler:

vitess/go/vt/vttablet/tabletserver/throttle/throttler.go

Lines 681 to 687 in 456296d

	case <-mysqlRefreshTicker.C:
	{
	// sparse
	if throttler.IsOpen() {
	go throttler.refreshMySQLInventory(ctx)
	}
	}

The function throttler.refreshMySQLInventory() should not be invoked with a goroutine; it must continue to run on same goroutine as the select clause, which is the single goroutine where all access to the throttler's fields is done.