[CommCoreModule] Pass setReadyState callback to ClientGetReadReactor

Summary: Rethought this a bit.. I previously thought multiple objects on the "network" thread would need to "hold on" to this function, but I think we can just pass it to ClientGetReadReactor which should be able to handle setting the readyState for all stages of the grpc stream request lifecycle.

Test Plan: Will be tested implicitly with subsequent diffs.

Reviewers: ashoat, def-au1t, palys-swm, karol-bisztyga

Reviewed By: ashoat

Subscribers: Adrian, benschac

Differential Revision: https://phabricator.ashoat.com/D3101