Corrected ApolloClient.queryManager typing as it may be undefined

[danilobuerger]

Found in apollographql/react-apollo#2164 that ApolloClient.queryManager can actually be undefined as it is lazily initialised. This PR corrects the typing to reflect this. I chose to model initQueryManager after initProxy as this pattern was already being used. I also considered adding a getter for queryManager to initialise it whenever called, but rejected it to keep the pattern of actively having to init the QueryManager.

// cc @rosskevin

[apollo-cla]

	Warnings
⚠️	There are library changes, but not tests. That's OK as long as you're refactoring existing code

Generated by 🚫 dangerJS

[danilobuerger]

@hwillson could you take a look at this please? I resolved all conflicts, again, it would be nice to get this merged.

[hwillson]

This looks great @danilobuerger - thanks very much!