Make max tip age an option instead of chainparam

After discussion in #7164 I think this is better.

Max tip age was introduced in #5987 to make it possible to run
testnet-in-a-box. But associating this behavior with the testnet chain
is wrong conceptually, as it is not needed in normal usage.
Should aim to make testnet test the software as-is.

Replace it with a (debug) option `-maxtipage`, which can be
specified only in the specific case.

src/init.cpp

@@ -456,6 +456,7 @@ std::string HelpMessage(HelpMessageMode mode)
         strUsage += HelpMessageOpt("-limitfreerelay=<n>", strprintf("Continuously rate-limit free transactions to <n>*1000 bytes per minute (default: %u)", DEFAULT_LIMITFREERELAY));
         strUsage += HelpMessageOpt("-relaypriority", strprintf("Require high priority for relaying free or low-fee transactions (default: %u)", DEFAULT_RELAYPRIORITY));
         strUsage += HelpMessageOpt("-maxsigcachesize=<n>", strprintf("Limit size of signature cache to <n> MiB (default: %u)", DEFAULT_MAX_SIG_CACHE_SIZE));
+        strUsage += HelpMessageOpt("-maxtipage=<n>", strprintf("Maximum tip age in seconds to consider node in initial block download (default: %u)", DEFAULT_MAX_TIP_AGE));
     }
     strUsage += HelpMessageOpt("-minrelaytxfee=<amt>", strprintf(_("Fees (in %s/kB) smaller than this are considered zero fee for relaying, mining and transaction creation (default: %s)"),
         CURRENCY_UNIT, FormatMoney(DEFAULT_MIN_RELAY_TX_FEE)));
@@ -994,6 +995,8 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
     if (GetBoolArg("-peerbloomfilters", true))
         nLocalServices |= NODE_BLOOM;
 
+    nMaxTipAge = GetArg("-maxtipage", DEFAULT_MAX_TIP_AGE);
+
     // ********************************************************* Step 4: application initialization: dir lock, daemonize, pidfile, debug log
 
     // Initialize elliptic curve code