Combine CCoinsViewCache's HaveCoins and const GetCoins into AccessCoins.

The efficient version of CCoinsViewCache::GetCoins only works for known-to-exist
cache entries, requiring a separate HaveCoins call beforehand. This is
inefficient as both perform a hashtable lookup.

Replace the non-mutable GetCoins with AccessCoins, which returns a potentially-NULL
pointer. This also decreases the overloading of GetCoins.

Also replace some copying (inefficient) GetCoins calls with equivalent AccessCoins,
decreasing the copying.