mempool: move interface into mempool package (#3524)

## Description

Refs #2659

Breaking changes in the mempool package:

[mempool] #2659 Mempool now an interface
    old Mempool renamed to CListMempool
    NewMempool renamed to NewCListMempool
    Option renamed to CListOption
    MempoolReactor renamed to Reactor
    NewMempoolReactor renamed to NewReactor
    unexpose TxID method
    TxInfo.PeerID renamed to SenderID
    unexpose MempoolReactor.Mempool

Breaking changes in the state package:

[state] #2659 Mempool interface moved to mempool package
    MockMempool moved to top-level mock package and renamed to Mempool

Non Breaking changes in the node package:

[node] #2659 Add Mempool method, which allows you to access mempool

## Commits

* move Mempool interface into mempool package

Refs #2659

Breaking changes in the mempool package:

- Mempool now an interface
- old Mempool renamed to CListMempool

Breaking changes to state package:

- MockMempool moved to mempool/mock package and renamed to Mempool
- Mempool interface moved to mempool package

* assert CListMempool impl Mempool

* gofmt code

* rename MempoolReactor to Reactor

- combine everything into one interface
- rename TxInfo.PeerID to TxInfo.SenderID
- unexpose MempoolReactor.Mempool

* move mempool mock into top-level mock package

* add a fixme

TxsFront should not be a part of the Mempool interface
because it leaks implementation details. Instead, we need to come up
with general interface for querying the mempool so the MempoolReactor
can fetch and broadcast txs to peers.

* change node#Mempool to return interface

* save commit = new reactor arch

* Revert "save commit = new reactor arch"

This reverts commit 1bfceacd9d.

* require CListMempool in mempool.Reactor

* add two changelog entries

* fixes after my own review

* quote interfaces, structs and functions

* fixes after Ismail's review

* make node's mempool an interface

* make InitWAL/CloseWAL methods a part of Mempool interface

* fix merge conflicts

* make node's mempool an interface

state/services.go

@@ -1,8 +1,6 @@
 package state
 
 import (
-	abci "github.com/tendermint/tendermint/abci/types"
-	"github.com/tendermint/tendermint/mempool"
 	"github.com/tendermint/tendermint/types"
 )
 
@@ -11,57 +9,6 @@ import (
 // NOTE: Interfaces used by RPC must be thread safe!
 //------------------------------------------------------
 
-//------------------------------------------------------
-// mempool
-
-// Mempool defines the mempool interface as used by the ConsensusState.
-// Updates to the mempool need to be synchronized with committing a block
-// so apps can reset their transient state on Commit
-type Mempool interface {
-	Lock()
-	Unlock()
-
-	Size() int
-	CheckTx(types.Tx, func(*abci.Response)) error
-	CheckTxWithInfo(types.Tx, func(*abci.Response), mempool.TxInfo) error
-	ReapMaxBytesMaxGas(maxBytes, maxGas int64) types.Txs
-	Update(int64, types.Txs, mempool.PreCheckFunc, mempool.PostCheckFunc) error
-	Flush()
-	FlushAppConn() error
-
-	TxsAvailable() <-chan struct{}
-	EnableTxsAvailable()
-}
-
-// MockMempool is an empty implementation of a Mempool, useful for testing.
-type MockMempool struct{}
-
-var _ Mempool = MockMempool{}
-
-func (MockMempool) Lock()     {}
-func (MockMempool) Unlock()   {}
-func (MockMempool) Size() int { return 0 }
-func (MockMempool) CheckTx(_ types.Tx, _ func(*abci.Response)) error {
-	return nil
-}
-func (MockMempool) CheckTxWithInfo(_ types.Tx, _ func(*abci.Response),
-	_ mempool.TxInfo) error {
-	return nil
-}
-func (MockMempool) ReapMaxBytesMaxGas(_, _ int64) types.Txs { return types.Txs{} }
-func (MockMempool) Update(
-	_ int64,
-	_ types.Txs,
-	_ mempool.PreCheckFunc,
-	_ mempool.PostCheckFunc,
-) error {
-	return nil
-}
-func (MockMempool) Flush()                        {}
-func (MockMempool) FlushAppConn() error           { return nil }
-func (MockMempool) TxsAvailable() <-chan struct{} { return make(chan struct{}) }
-func (MockMempool) EnableTxsAvailable()           {}
-
 //------------------------------------------------------
 // blockstore
 
@@ -96,7 +43,7 @@ type EvidencePool interface {
 	IsCommitted(types.Evidence) bool
 }
 
-// MockMempool is an empty implementation of a Mempool, useful for testing.
+// MockEvidencePool is an empty implementation of a Mempool, useful for testing.
 type MockEvidencePool struct{}
 
 func (m MockEvidencePool) PendingEvidence(int64) []types.Evidence { return nil }