mempool: Pass in -mempoolexpiry instead of referencing gArgs

- Store the mempool expiry (-mempoolexpiry) in CTxMemPool as a
  std::chrono::seconds member.

- Remove the requirement to explicitly specify a mempool expiry for
  LimitMempoolSize(...), just use the newly-introduced member.

- Remove all now-unnecessary instances of:
    std::chrono::hours{gArgs.GetIntArg("-mempoolexpiry", DEFAULT_MEMPOOL_EXPIRY)}

src/kernel/mempool_options.h

@@ -4,12 +4,15 @@
 #ifndef BITCOIN_KERNEL_MEMPOOL_OPTIONS_H
 #define BITCOIN_KERNEL_MEMPOOL_OPTIONS_H
 
+#include <chrono>
 #include <cstdint>
 
 class CBlockPolicyEstimator;
 
 /** Default for -maxmempool, maximum megabytes of mempool memory usage */
 static constexpr unsigned int DEFAULT_MAX_MEMPOOL_SIZE_MB{300};
+/** Default for -mempoolexpiry, expiration time for mempool transactions in hours */
+static constexpr unsigned int DEFAULT_MEMPOOL_EXPIRY{336};
 
 namespace kernel {
 /**
@@ -25,6 +28,7 @@ struct MemPoolOptions {
     /* The ratio used to determine how often sanity checks will run.  */
     int check_ratio{0};
     int64_t max_size_bytes{DEFAULT_MAX_MEMPOOL_SIZE_MB * 1'000'000};
+    std::chrono::seconds expiry{std::chrono::hours{DEFAULT_MEMPOOL_EXPIRY}};
 };
 } // namespace kernel