DumpMempool: Pass in dump_path, stop using gArgs

Also introduce node::{ShouldPersistMempool,MempoolPath} helper functions
in node/mempool_persist_args.{h,cpp} which are used by non-kernel
DumpMempool callers to determine whether or not to automatically dump
the mempool and where to dump it to.

src/Makefile.am

@@ -198,6 +198,7 @@ BITCOIN_CORE_H = \
   node/chainstate.h \
   node/coin.h \
   node/context.h \
+  node/mempool_persist_args.h \
   node/miner.h \
   node/minisketchwrapper.h \
   node/psbt.h \
@@ -380,6 +381,7 @@ libbitcoin_node_a_SOURCES = \
   node/context.cpp \
   node/eviction.cpp \
   node/interfaces.cpp \
+  node/mempool_persist_args.cpp \
   node/miner.cpp \
   node/minisketchwrapper.cpp \
   node/psbt.cpp \