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.

Partial backport of Core PR16194