Fix bugs handling client UTXO requests

Allow strings for ints - Electrum command line doesn't convert
Don't blow away hash168s from the DB

server/block_processor.py

@@ -767,7 +767,8 @@ class BlockProcessor(LoggedClass):
         '''Returns the hash168 for a UTXO.'''
         hash168 = None
         if 0 <= index <= 65535:
-            hash168 = self.utxo_cache(tx_hash, struct.pack('<H', index))
+            idx_packed = struct.pack('<H', index)
+            hash168 = self.utxo_cache.hash168(tx_hash, idx_packed)
             if hash168 == NO_CACHE_ENTRY:
                 hash168 = None
         return hash168

server/cache.py

@@ -109,7 +109,7 @@ class UTXOCache(LoggedClass):
             return value
 
         # Oh well.  Find and remove it from the DB.
-        hash168 = self.hash168(prev_hash, idx_packed)
+        hash168 = self.hash168(prev_hash, idx_packed, True)
         if not hash168:
             return NO_CACHE_ENTRY
 
@@ -144,7 +144,7 @@ class UTXOCache(LoggedClass):
 
         raise Exception('could not resolve UTXO key collision')
 
-    def hash168(self, tx_hash, idx_packed):
+    def hash168(self, tx_hash, idx_packed, delete=False):
         '''Return the hash168 paid to by the given TXO.
 
         Refers to the database.  Returns None if not found (which is
@@ -158,7 +158,8 @@ class UTXOCache(LoggedClass):
             return None
 
         if len(data) == 25:
-            self.cache_delete(key)
+            if delete:
+                self.cache_delete(key)
             return data[:21]
 
         assert len(data) % 25 == 0
@@ -168,7 +169,8 @@ class UTXOCache(LoggedClass):
             (tx_num, ) = struct.unpack('<I', data[n+21:n+25])
             my_hash, height = self.parent.get_tx_hash(tx_num)
             if my_hash == tx_hash:
-                self.cache_write(key, data[:n] + data[n+25:])
+                if delete:
+                    self.cache_write(key, data[:n] + data[n+25:])
                 return data[n:n+21]
 
         raise Exception('could not resolve hash168 collision')

server/protocol.py

@@ -143,8 +143,14 @@ class JSONRPC(asyncio.Protocol, LoggedClass):
 
     @classmethod
     def non_negative_integer_from_param(cls, param):
-        if isinstance(param, int) and param >= 0:
-            return param
+        try:
+            param = int(param)
+        except ValueError:
+            pass
+        else:
+            if param >= 0:
+                return param
+
         raise RPCError('param should be a non-negative integer: {}'
                        .format(param))