mixing: add mixpool package

The mixpool package implements a memory pool of recently observed mix
messages.  Similar to the transaction mempool, the mixpool allows these
messages to be temporarily stored in memory to be relayed through the P2P
network.  It handles message acceptance, expiry, UTXO ownership proof checks,
and that previously referenced messages have also been accepted to the
mixpool.

The mixpool is designed with both full-node and wallet usage in mind,
providing all of these same acceptance rules to mixing wallets with the
exception of UTXO proof checks.  For wallets, it also implements query
functions for messages matching compatible pairings and messages belong to
ongoing sessions.

mixing/mixpool/log.go

@@ -0,0 +1,31 @@
+// Copyright (c) 2020-2021 The Decred developers
+// Use of this source code is governed by an ISC
+// license that can be found in the LICENSE file.
+
+package mixpool
+
+import (
+	"github.com/decred/slog"
+)
+
+// log is a logger that is initialized with no output filters.  This
+// means the package will not perform any logging by default until the caller
+// requests it.
+// The default amount of logging is none.
+//var log = slog.Disabled
+
+// UseLogger uses a specified Logger to output package logging info.
+// This should be used in preference to SetLogWriter if the caller is also
+// using slog.
+func UseLogger(logger slog.Logger) {
+	//	log = logger
+}
+
+// pickNoun returns the singular or plural form of a noun depending on the
+// provided count.
+func pickNoun(n uint64, singular, plural string) string {
+	if n == 1 {
+		return singular
+	}
+	return plural
+}