Move DEFAULT_PERSIST_MEMPOOL out of libbitcoinkernel

It is no longer used by anything inside libbitcoinkernel, move it to
node/mempool_persist_args.h where it belongs.

src/init.cpp

@@ -109,6 +109,7 @@ using node::CalculateCacheSizes;
 using node::ChainstateLoadVerifyError;
 using node::ChainstateLoadingError;
 using node::CleanupBlockRevFiles;
+using node::DEFAULT_PERSIST_MEMPOOL;
 using node::DEFAULT_PRINTPRIORITY;
 using node::DEFAULT_STOPAFTERBLOCKIMPORT;
 using node::LoadChainstate;