[refactor] Make signals optional in mempool and chainman

This is done in preparation for the next two commits, where the
CMainSignals are de-globalized.

This avoids adding new constructor arguments to the ChainstateManager
and CTxMemPool classes over the next two commits.

This could also allow future tests that are only interested in the
internal behaviour of the classes to forgo instantiating the signals.

src/kernel/mempool_options.h

@@ -13,6 +13,8 @@
 #include <cstdint>
 #include <optional>
 
+class CMainSignals;
+
 /** Default for -maxmempool, maximum megabytes of mempool memory usage */
 static constexpr unsigned int DEFAULT_MAX_MEMPOOL_SIZE_MB{300};
 /** Default for -maxmempool when blocksonly is set */
@@ -56,6 +58,8 @@ struct MemPoolOptions {
     bool full_rbf{DEFAULT_MEMPOOL_FULL_RBF};
     bool persist_v1_dat{DEFAULT_PERSIST_V1_DAT};
     MemPoolLimits limits{};
+
+    CMainSignals* signals{nullptr};
 };
 } // namespace kernel