[R4R] Add adr-034: PrivValidator file structure (#2751)

* add adr-034

* update changelog

* minor changes

* do some refactor

CHANGELOG.md

@@ -134,6 +134,8 @@ increasing attention to backwards compatibility. Thanks for bearing with us!
 - [abci] [\#2557](https://github.com/tendermint/tendermint/issues/2557) Add `Codespace` field to `Response{CheckTx, DeliverTx, Query}`
 - [abci] [\#2662](https://github.com/tendermint/tendermint/issues/2662) Add `BlockVersion` and `P2PVersion` to `RequestInfo`
 - [crypto/merkle] [\#2298](https://github.com/tendermint/tendermint/issues/2298) General Merkle Proof scheme for chaining various types of Merkle trees together
+- [docs/architecture] [\#1181](https://github.com/tendermint/tendermint/issues/1181) S
+plit immutable and mutable parts of priv_validator.json
 
 ### IMPROVEMENTS:
 - Additional Metrics

docs/architecture/adr-034-priv-validator-file-structure.md

@@ -0,0 +1,72 @@
+# ADR 034: PrivValidator file structure
+
+## Changelog
+
+03-11-2018: Initial Draft
+
+## Context
+
+For now, the PrivValidator file `priv_validator.json` contains mutable and immutable parts. 
+Even in an insecure mode which does not encrypt private key on disk, it is reasonable to separate 
+the mutable part and immutable part.
+
+References:
+[#1181](https://github.com/tendermint/tendermint/issues/1181)
+[#2657](https://github.com/tendermint/tendermint/issues/2657)
+[#2313](https://github.com/tendermint/tendermint/issues/2313)
+
+## Proposed Solution
+
+We can split mutable and immutable parts with two structs:
+```go
+// FilePVKey stores the immutable part of PrivValidator
+type FilePVKey struct {
+	Address types.Address  `json:"address"`
+	PubKey  crypto.PubKey  `json:"pub_key"`
+	PrivKey crypto.PrivKey `json:"priv_key"`
+
+	filePath string
+}
+
+// FilePVState stores the mutable part of PrivValidator
+type FilePVLastSignState struct {
+	Height    int64        `json:"height"`
+	Round     int          `json:"round"`
+	Step      int8         `json:"step"`
+	Signature []byte       `json:"signature,omitempty"`
+	SignBytes cmn.HexBytes `json:"signbytes,omitempty"`
+
+	filePath string
+	mtx      sync.Mutex
+}
+```
+
+Then we can combine `FilePVKey` with `FilePVLastSignState` and will get the original `FilePV`.
+
+```go
+type FilePV struct {
+	Key           FilePVKey
+	LastSignState FilePVLastSignState
+}
+```
+
+As discussed, `FilePV` should be located in `config`, and `FilePVLastSignState` should be stored in `data`. The 
+store path of each file should be specified in `config.yml`.
+
+What we need to do next is changing the methods of `FilePV`.
+
+## Status
+
+Draft.
+
+## Consequences
+
+### Positive
+
+- separate the mutable and immutable of PrivValidator
+
+### Negative
+
+- need to add more config for file path
+
+### Neutral