[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/init.cpp

@@ -1449,6 +1449,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
         .chainparams = chainparams,
         .datadir = args.GetDataDirNet(),
         .notifications = *node.notifications,
+        .signals = &GetMainSignals(),
     };
     Assert(ApplyArgsManOptions(args, chainman_opts)); // no error can happen, already checked in AppInitParameterInteraction
 
@@ -1478,6 +1479,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
 
     CTxMemPool::Options mempool_opts{
         .check_ratio = chainparams.DefaultConsistencyChecks() ? 1 : 0,
+        .signals = &GetMainSignals(),
     };
     auto result{ApplyArgsManOptions(args, chainparams, mempool_opts)};
     if (!result) {