txdb: add CCoinsViewDB::ChangeCacheSize

We'll need this to dynamically update the cache size of the existing
CCoinsViewDB instance when we create a new one during snapshot activation.

This requires us to keep the CDBWrapper instance as a pointer instead of
a reference so that we're able to destruct it and create a new instance
when the cache size changes.

Also renames `db` to `m_db` since we're already modifying each usage.

Includes feedback from Russ Yanofsky.

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