Separate out the management of chain-agnostic block metadata from any given
CChainState instance. This allows us to avoid duplicating data like
`mapBlockIndex` unnecessarily for multiple chainstates.
This also adds a CChainState constructor that accepts and sets m_blockman.
Ultimately this reference will point to a BlockMan instance that
is shared across CChainStates.
This commit can be decomposed into smaller commits if necessary.
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Partial backport of Core PR16194