liteclient signature check support

1. update liteclient/liteserver. Now liteserver sends signatures of
blocks and liteclient checks them. I.e. liteclient completely checks
received data.
2. validator-engine: more GC options
3. blockchain-explorer: show all block transactions (instead of 256)
4. some bugfixes

common/errorcode.h

@@ -20,6 +20,14 @@
 
 namespace ton {
 
-enum ErrorCode : int { failure = 601, error = 602, warning = 603, protoviolation = 621, notready = 651, timeout = 652 };
+enum ErrorCode : int {
+  failure = 601,
+  error = 602,
+  warning = 603,
+  protoviolation = 621,
+  notready = 651,
+  timeout = 652,
+  cancelled = 653
+};
 
 }