Enable high-priority alerts to put the RPC into safe mode

This reverts the changes in 986b5e257e and adds a
priority check.

Closes #1106

src/main.h

@@ -57,6 +57,8 @@ static const unsigned int DEFAULT_BLOCK_MIN_SIZE = 0;
 static const unsigned int DEFAULT_BLOCK_PRIORITY_SIZE = 50000;
 /** Default for accepting alerts from the P2P network. */
 static const bool DEFAULT_ALERTS = true;
+/** Minimum alert priority for enabling safe mode. */
+static const int ALERT_PRIORITY_SAFE_MODE = 4000;
 /** The maximum size for transactions we're willing to relay/mine */
 static const unsigned int MAX_STANDARD_TX_SIZE = 100000;
 /** Maximum number of signature check operations in an IsStandard() P2SH script */