Make stale_reads_ok an argument of __init__

Actually, stale_reads_ok is already an argument, but for some
reason it was not used anywhere in the code. So, just use it.
We borrow the existing code from the object server, which uses
setdefault(). We don't really need it, and could simply use
kwargs['stale_reads_ok']=stale_reads_ok, but let's keep the
divergencies down.

This was prompted by the desire to document the API of DB Broker
for the LFS work, and finding that documenting magical assignments
was a pain.

Change-Id: Id8f83358ad7709f0df826fbc520b3dfba026a2f1
Merge "Make stale_reads_ok an argument of __init__"