state: txindex/kv: fsync data to disk immediately after receiving it (#4104)

## Issue

Why this pr:
When restarting chain node, sometimes we lost tx index about recent(around 80)blocks, and some client complains that they can't find the tx by RPC call(tx_search) when the tx do exist in the block.

I try to partially fix this issue in a simple way by writing the index data in a sync way.
There is no performance difference under 1K TPS according to our test.

It is still possible that lost index data after restarting the node, but only 2 block data will lost at most.
I try to totally fix this in https://github.com/tendermint/tendermint/pull/3847/files, but this one is simple and can solve most part of the issue. Please review first, thks.

## Comments

Anton:

BEFORE:

BenchmarkTxIndex1-2               100000             12434 ns/op
BenchmarkTxIndex500-2                300           5151564 ns/op
BenchmarkTxIndex1000-2               100          15053910 ns/op
BenchmarkTxIndex2000-2               100          18238892 ns/op
BenchmarkTxIndex10000-2               20         124287930 ns/op

AFTER:

BenchmarkTxIndex1-2                 2000            795431 ns/op
BenchmarkTxIndex500-2                200           6385124 ns/op
BenchmarkTxIndex1000-2               100          11388219 ns/op
BenchmarkTxIndex2000-2               100          20514873 ns/op
BenchmarkTxIndex10000-2               20         107456004 ns/op

Performance drop is pretty steep, but I think it's the right thing to do UNTIL we have a WAL.

CHANGELOG_PENDING.md

@@ -30,3 +30,4 @@ program](https://hackerone.com/tendermint).
 ### BUG FIXES:
 
 - [tools] [\#4023](https://github.com/tendermint/tendermint/issues/4023) Refresh `tm-monitor` health when validator count is updated (@erikgrinaker)
+- [state] [\#4104](https://github.com/tendermint/tendermint/pull/4104) txindex/kv: Fsync data to disk immediately after receiving it (@guagualvcha)

state/txindex/kv/kv.go

@@ -103,7 +103,7 @@ func (txi *TxIndex) AddBatch(b *txindex.Batch) error {
 		storeBatch.Set(hash, rawBytes)
 	}
 
-	storeBatch.Write()
+	storeBatch.WriteSync()
 	return nil
 }
 
@@ -132,7 +132,7 @@ func (txi *TxIndex) Index(result *types.TxResult) error {
 	}
 
 	b.Set(hash, rawBytes)
-	b.Write()
+	b.WriteSync()
 
 	return nil
 }