Trivial journal for private transactions (#10056)

* Journal for private txs added

* Tests after adding logging to private tx fixed

* Logs getter and tests added

* Time and amount limit for logs added

* RPC method for log retrieving added

* Correct path name and time validation implemented

* References for parameters added, redundant cloning reworked

* References for parameters added, redundant cloning reworked

* Work with json moved to the separate struct

* Serialization test added

* Fixed build after the merge with head

* Documentation for methods fixed, redundant field removed

* Fixed error usages

* Timestamp trait implemented for std struct

* Commented code removed

* Remove timestamp source, rework serialization test

* u64 replaced with SystemTime

* Path made mandatory for logging

* Source of monotonic time added

* into_system_time method renamed

* Initialize time source by max from current system time and max creation time from already saved logs

* Redundant conversions removed, code a little bit reworked according to review comments

* One more redundant conversion removed, rpc call simplified

ethcore/sync/src/tests/private.rs

@@ -69,11 +69,13 @@ fn send_private_transaction() {
 	let validator_config = ProviderConfig{
 		validator_accounts: vec![s1.address()],
 		signer_account: None,
+		logs_path: None,
 	};
 
 	let signer_config = ProviderConfig{
 		validator_accounts: Vec::new(),
 		signer_account: Some(s0.address()),
+		logs_path: None,
 	};
 
 	let private_keys = Arc::new(StoringKeyProvider::default());