Separate resolvers for each operation

[pavelnikolov]

This PR allows having fields in the schema with the same name in different operations. For example, consider the schema below:

schema {
	query: Query
	mutation: Mutation
	subscription: Subscription
}

type Query {
	hello: String!
}

type Mutation {
	hello: String!
}

type Subscription {
	hello: HelloEvent!
}

type HelloEvent {
	msg: String!
}

Before this PR it was not possible to have more than one operation with the same field name. This PR resolves the problem in a backwards compatible way.
Continuation from #182
Fixes #145
Fixes #562