Invoke subgraph handler in watcher event processing (#34)

* Invoke subgraph handler in watcher event processing

* Fix error when invoking subgraph handler

* Parse events using event signature specified in subgraph yaml

* Use contract abi to parse event params

* Invoke event handler based on event signature

* Fill event with block and transaction data

* Comment missing fields in block and transaction data

packages/graph-node/test/subgraph/example1/schema.graphql

@@ -2,5 +2,5 @@ type ExampleEntity @entity {
   id: ID!
   count: BigInt!
   param1: String! # string
-  param2: BigInt! # uint256
+  param2: Int! # uint8
 }