Add an upwards reference to chainstate instances to the owning
ChainstateManager. This is necessary because there are a number
of `this_chainstate == chainman.ActiveChainstate()` checks that
will happen (as a result of assumeutxo) in functions that otherwise
don't have an easily-accessible reference to the chainstate's
ChainManager.
Since we now have multiple chainstate objects, only one of them is active at any given
time. An active chainstate has a mempool, but there's no point to others having one.
This change will simplify proposed assumeutxo semantics. See the discussion here:
https://github.com/bitcoin/bitcoin/pull/15606#pullrequestreview-692965905
Co-authored-by: Russell Yanofsky <russ@yanofsky.org>
Also adds CCoinsViewCache::ReallocateCache() to attempt to free
memory that the cacheCoins's allocator may be hanging onto when
downsizing the cache.
Adds `CChainState::m_coins{tip,db}_cache_size_bytes` data members
so that we can reference cache size on a per-chainstate basis for
flushing.
