mixing: introduce module

This introduces the mixing module and mixpool package which implements
a mixing message pool.  Similar to the transaction mempool, the
mixpool records recently observed mixing messages and 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 also designed with wallet usage in mind, providing most
of these same acceptance rules to mixing messages received by wallets,
and implements queries for messages matching compatible pairings and
ongoing sessions.

mixing/go.mod

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+module github.com/decred/dcrd/mixing
+
+go 1.17

mixing/mixpool/log.go

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+// 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
+}