add state sync reactor (#4705)

Fixes #828. Adds state sync, as outlined in [ADR-053](https://github.com/tendermint/tendermint/blob/master/docs/architecture/adr-053-state-sync-prototype.md). See related PRs in Cosmos SDK (https://github.com/cosmos/cosmos-sdk/pull/5803) and Gaia (https://github.com/cosmos/gaia/pull/327).

This is split out of the previous PR #4645, and branched off of the ABCI interface in #4704. 

* Adds a new P2P reactor which exchanges snapshots with peers, and bootstraps an empty local node from remote snapshots when requested.

* Adds a new configuration section `[statesync]` that enables state sync and configures the light client. Also enables `statesync:info` logging by default.

* Integrates state sync into node startup. Does not support the v2 blockchain reactor, since it needs some reorganization to defer startup.

statesync/mocks/state_provider.go

@@ -0,0 +1,82 @@
+// Code generated by mockery v1.0.0. DO NOT EDIT.
+
+package mocks
+
+import (
+	mock "github.com/stretchr/testify/mock"
+	state "github.com/tendermint/tendermint/state"
+
+	types "github.com/tendermint/tendermint/types"
+)
+
+// StateProvider is an autogenerated mock type for the StateProvider type
+type StateProvider struct {
+	mock.Mock
+}
+
+// AppHash provides a mock function with given fields: height
+func (_m *StateProvider) AppHash(height uint64) ([]byte, error) {
+	ret := _m.Called(height)
+
+	var r0 []byte
+	if rf, ok := ret.Get(0).(func(uint64) []byte); ok {
+		r0 = rf(height)
+	} else {
+		if ret.Get(0) != nil {
+			r0 = ret.Get(0).([]byte)
+		}
+	}
+
+	var r1 error
+	if rf, ok := ret.Get(1).(func(uint64) error); ok {
+		r1 = rf(height)
+	} else {
+		r1 = ret.Error(1)
+	}
+
+	return r0, r1
+}
+
+// Commit provides a mock function with given fields: height
+func (_m *StateProvider) Commit(height uint64) (*types.Commit, error) {
+	ret := _m.Called(height)
+
+	var r0 *types.Commit
+	if rf, ok := ret.Get(0).(func(uint64) *types.Commit); ok {
+		r0 = rf(height)
+	} else {
+		if ret.Get(0) != nil {
+			r0 = ret.Get(0).(*types.Commit)
+		}
+	}
+
+	var r1 error
+	if rf, ok := ret.Get(1).(func(uint64) error); ok {
+		r1 = rf(height)
+	} else {
+		r1 = ret.Error(1)
+	}
+
+	return r0, r1
+}
+
+// State provides a mock function with given fields: height
+func (_m *StateProvider) State(height uint64) (state.State, error) {
+	ret := _m.Called(height)
+
+	var r0 state.State
+	if rf, ok := ret.Get(0).(func(uint64) state.State); ok {
+		r0 = rf(height)
+	} else {
+		r0 = ret.Get(0).(state.State)
+	}
+
+	var r1 error
+	if rf, ok := ret.Get(1).(func(uint64) error); ok {
+		r1 = rf(height)
+	} else {
+		r1 = ret.Error(1)
+	}
+
+	return r0, r1
+}