replace the external pointer `phashBlock` with an owned `m_block_hash`
member in CBlockIndex. Previously the hash was stored in the BlockMap
key and CBlockIndex held a pointer to it, creating a lifetime dependency
on the map's stable addressing. Storing the hash directly makes
CBlockIndex self-contained, which is a prerequisite for moving to a
copy-on-write BlockMap with shared_ptr<CBlockIndex>.

Add a new constructor CBlockIndex(const uint256& hash) and delegate the
existing CBlockHeader constructor to it. Remove all phashBlock
assignemnts and replace with m_block_hash.

Replace the previous BlockMap (unordered_map<uint256, CBlockIndex>) with
a new copy-on-write BlockMap that stores shared_pointer<CBlockIndex>.
This is a prefactor for lock-free block index reads.

The new BlockMap uses a two-tier structure (stable + recent) backed by
stlab::copy-on-write. New entries are inserted into a small recent map
and promoted to the stable COW map when the recent map exceeds a
threshold. Copies of the BlockMap are cheap (refcount bump) and produce
consistent point-in-time snapshots.

Call sites are updated from value semantics (&senty.second) to pointer
semantica (entry.second).

No locking changes in this commit; cs_main guards are preserved as-is.

LookupBlockIndex and GetAllBlockIndices now take a snapshot copy instead
of requiring cs_main.

Add m_block_index_mutex to serialize writes in AddToBlockIndex and
InsertBlockIndex. Split AddToBlockIndex into separate lock scopes so
m_block_index_mutex and cs_main are not held simultaneously.

Remove cs_main locks that are no longer needed when calling
BlockManager::LookupBlockIndex.

Changes:
- Remove cs_main locks from index/base.cpp, kernel/bitcoinkernel.cpp,
  net_processing.cpp, node/interfaces.cpp, node/miner.cpp,
  rpc/gettxoutproof.cpp, rpc/blockchain.cpp and rest.cpp
- Remove unused validation.h and sync.h includes from
  kernel/coinstats.cpp
- Update expected circular dependency path for kernel/coinstats due to
  removal of validation.h include

Now that CChain handles its own thread safety internally via m_mutex,
callers no longer need to hold cs_main when accessing chain data.
Removes the redundant LOCK(cs_main) and WITH_LCOK(cs_main) annotations
from call sites across RPC, indexes, node interfaces, REST and tests.

Replace cs_main lock with targeted locking and use BlockIndexSnapshot()
to iterate over the block index.

…ll sites