Clean unconfirmed transactions

Invalid/expired unconfirmed transactions are cleaned during various calls,
e.g. requesting list of unconfirmed transactions,
or requesting account's last reference (including considering unconfirmed),
or generating a new block.

BlockGenerator now calls repository.discardChanges before sleep to
release any repository-level locks.

Added settings.json toggle "wipeUnconfirmedOnStart" (default: true)
to aid testing.

REMOVED API call /addresses/lastreference/{address}/unconfirmed as
/addresses/lastreference/{address} now considers unconfirmed
transactions regardless.

Added useful error to /transactions/sign if an invalid private key
is supplied.

Improved API "invalid transaction" error to include actual apsect
that caused validity check to fail (e.g. invalid reference)

src/main/resources/i18n/ApiError_en.properties

@@ -33,6 +33,8 @@ WALLET_NOT_IN_SYNC=wallet needs to be synchronized
 INVALID_NETWORK_ADDRESS=invalid network address
 ADDRESS_NO_EXISTS=account address does not exist
 INVALID_CRITERIA=invalid search criteria
+INVALID_REFERENCE=invalid reference
+INVALID_PRIVATE_KEY=invalid private key
 
 # Wallet
 WALLET_NO_EXISTS=wallet does not exist
@@ -47,7 +49,7 @@ BLOCK_NO_EXISTS=block does not exist
 # Transactions
 TRANSACTION_NO_EXISTS=transaction does not exist
 PUBLIC_KEY_NOT_FOUND=public key not found
-TRANSACTION_INVALID=transaction invalid
+TRANSACTION_INVALID=transaction invalid: %s
 
 # Names
 NAME_NO_EXISTS=name does not exist