## 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.