mixing: Add mixpool package.

[jrick]

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.

Requires #3207.

[davecgh]

I think almost all of this should be an internal package, just like mempool. Since mixing is a development module for the time being, it's fine, but ultimately I really don't want to go back to having a huge surface of exposed code that really is specifically for the server itself.

I expect that some of it the code should be shared, akin to how blockchain/standalone does it, but pretty much everything in mixpool.go, mixpool_test.go, and log.go, at least, should be internal.

[jrick]

I think almost all of this should be an internal package, just like mempool. Since mixing is a development module for the time being, it's fine, but ultimately I really don't want to go back to having a huge surface of exposed code that really is specifically for the server itself.

I expect that some of it the code should be shared, akin to how blockchain/standalone does it, but pretty much everything in mixpool.go, mixpool_test.go, and log.go, at least, should be internal.

Not possible with current design.

$ pwd                  
/home/jrick/src/dcrwallet
$ find . -name '*.go' | xargs grep mixpool.Pool
./wallet/mixing.go:func (w *mixingWallet) Mixpool() *mixpool.Pool {
./wallet/wallet.go:	mixpool   *mixpool.Pool

This is because the mixclient package expects the wallet interface to provide an accessor function for the mixpool.