[draft] CCoinsView interface separation

[optout21]

EXPERIMENTAL

Separation of CCoinsView interface into readonly and write parts.

• Split CCoinsView into ...ReadOnly, ...ReadCacheMutable, and ...Write, built on top of each other. Note: the Read version may not be useful/needed in itself.
• In most places, the ...Write version is used, but in some places usage can be restricted to non-writeable interface. Non-write usages: ComputeUTXOStats, RPC rest_getutxos, GetUTXOStats, CalculatePrevHeights, CalculateLockPointsAtTip, VerifyDB.
• Split CCoinsViewBacked into ...ReadCacheMutable and ...Write (ReadOnly is not required). For now use the ...Write version -- ...ReadCacheMutable could be used in a few places, like the ErrorCatcher and Mempool, but they are sometimes used as a backend for Cached, which requires Write.
• Carve out non-write CCoinsViewCacheRead from CCoinsViewCache

References
Coins Cache Cleanup checklist 34280