By Trevor Hunnicutt (Reuters) - BlackRock on Tuesday informed staff that it was reshuffling the managers of some of its most important investment units, including its stock and bond-picking businesses, according to an internal memo seen by Reuters. The world's largest asset manager said it would unify its global fixed-income business under Tim Webb, a longtime manager at the firm, and Rick Rieder, a well-known investor who is becoming chief investment officer of global fixed income. New York-based BlackRock is also moving a senior executive, Rich Kushel, into a position as head of multi-asset strategies, which includes products that straddle...