[lib][native][web] useNewThread -> useNewThinThread

Summary:
Renaming this keyserver call to distinguish from thick threads, which are initialized through Olm session over Tunnelbroker.

Depends on D12484

Test Plan: Flow

Reviewers: kamil, marcin

Reviewed By: kamil

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12485