mining: store block create options in NodeContext

Read configured mining options once during startup instead of parsing
options like -blockmaxweight each time a block template is generated.

Store the parsed startup options as BlockCreateOptions. Members left unset
keep representing unset options; hardcoded defaults are applied by
FlattenMiningOptions() before the options are used. IPC overrides and node
defaults can then be merged with the same option type used by
BlockAssembler.

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

src/node/mining_args.cpp

@@ -13,10 +13,12 @@
 #include <policy/policy.h>
 #include <tinyformat.h>
 #include <util/moneystr.h>
+#include <util/result.h>
 #include <util/translation.h>
 
 #include <cstdint>
 #include <optional>
+#include <string>
 #include <utility>
 
 using common::AmountErrMsg;