[CommCoreModule] Call onopen and onclose callback from ClientGetReadReactor

Summary: Execute (well queue up on JS event thread) the onopen callback when the OnReadInitialMetadataDone lifecycle event is hit by gRPC. Do the same for onclose callback when the OnDone lifecycle event is hit.

Test Plan:

1. Call openSocket
2. Assign .onopen callback
3. Assign .onclose callback
4. Make sure .onopen callback fires (console log something)
5. Call socket.close()
6. Make sure the .onclose callback fires (console log something)

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

Reviewed By: ashoat, karol-bisztyga

Subscribers: Adrian, benschac

Differential Revision: https://phabricator.ashoat.com/D3123