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/validation.h

@@ -59,8 +59,6 @@ namespace Consensus {
 struct Params;
 } // namespace Consensus
 
-/** Default for -mempoolexpiry, expiration time for mempool transactions in hours */
-static const unsigned int DEFAULT_MEMPOOL_EXPIRY = 336;
 /** Maximum number of dedicated script-checking threads allowed */
 static const int MAX_SCRIPTCHECK_THREADS = 15;
 /** -par default (number of script-checking threads, 0 = auto) */