[lib] Modify getClientResponsesSelector to return Promise

Summary: In order to get the signedIdentityKeysBlob, we're going to need to await commCoreModule.getUserPublicKey. As a first step, we refactor getClientResponsesSelector so we'll be able to await getUserPublicKey inside.

Test Plan:
Set breakpoints and ensure that native/web send responses as expected and keyserver continues to get responses as expected. Will also be tested implicitly by subsequent diffs.

Haven't done this yet, will test before landing.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D7034