Improve logging in mock-request
Improve logging in mock-request

Try to make it a bit more obvious what's been going on in mock-request: add
timestamps to lines, and an identifier so that where we have two flushes in
parallel, we can see what's what.

vector-im/riot-web,vector-im/riot-web,vector-im/vector-web,vector-im/vector-web,vector-im/vector-web,vector-im/riot-web,vector-im/riot-web,vector-im/riot-web,vector-im/vector-web,vector-im/riot-web,martindale/vector,martindale/vector,vector-im/vector-web