Add ChainstateManager::m_adjusted_time_callback

This decouples validation.cpp from netaddress.cpp (transitively,
timedata.cpp, and asmap.cpp).

This is important for libbitcoinkernel as:

- There is no reason for the consensus engine to be coupled with
  netaddress, timedata, and asmap
- Users of libbitcoinkernel can now easily supply their own
  std::function that provides the adjusted time.

See the src/Makefile.am changes for some satisfying removals.

src/kernel/chainstatemanager_opts.h

@@ -17,6 +17,7 @@ class CChainParams;
  */
 struct ChainstateManagerOpts {
     const CChainParams& chainparams;
+    const std::function<int64_t()> adjusted_time_callback{nullptr};
 };
 
 #endif // BITCOIN_KERNEL_CHAINSTATEMANAGER_OPTS_H